PL/pgSQL MDASH SQL Procedural Language {#plpgsql}
======================================

PL/pgSQL

Overview {#plpgsql-overview}
========

PL/pgSQL is a loadable procedural language for the PostgreSQL database
system. The design goals of PL/pgSQL were to create a loadable
procedural language that

-   can be used to create functions, procedures, and triggers,

-   adds control structures to the SQL language,

-   can perform complex computations,

-   inherits all user-defined types, functions, procedures, and
    operators,

-   can be defined to be trusted by the server,

-   is easy to use.

Functions created with PL/pgSQL can be used anywhere that built-in
functions could be used. For example, it is possible to create complex
conditional computation functions and later use them to define operators
or use them in index expressions.

In PostgreSQL 9.0 and later, PL/pgSQL is installed by default. However
it is still a loadable module, so especially security-conscious
administrators could choose to remove it.

Advantages of Using PL/pgSQL {#plpgsql-advantages}
----------------------------

SQL is the language PostgreSQL and most other relational databases use
as query language. It\'s portable and easy to learn. But every SQL
statement must be executed individually by the database server.

That means that your client application must send each query to the
database server, wait for it to be processed, receive and process the
results, do some computation, then send further queries to the server.
All this incurs interprocess communication and will also incur network
overhead if your client is on a different machine than the database
server.

With PL/pgSQL you can group a block of computation and a series of
queries *inside* the database server, thus having the power of a
procedural language and the ease of use of SQL, but with considerable
savings of client/server communication overhead.

-   Extra round trips between client and server are eliminated

-   Intermediate results that the client does not need do not have to be
    marshaled or transferred between server and client

-   Multiple rounds of query parsing can be avoided

This can result in a considerable performance increase as compared to an
application that does not use stored functions.

Also, with PL/pgSQL you can use all the data types, operators and
functions of SQL.

Supported Argument and Result Data Types {#plpgsql-args-results}
----------------------------------------

Functions written in PL/pgSQL can accept as arguments any scalar or
array data type supported by the server, and they can return a result of
any of these types. They can also accept or return any composite type
(row type) specified by name. It is also possible to declare a PL/pgSQL
function as accepting `record`, which means that any composite type will
do as input, or as returning `record`, which means that the result is a
row type whose columns are determined by specification in the calling
query, as discussed in [???](#queries-tablefunctions).

PL/pgSQL functions can be declared to accept a variable number of
arguments by using the `VARIADIC` marker. This works exactly the same
way as for SQL functions, as discussed in
[???](#xfunc-sql-variadic-functions).

PL/pgSQL functions can also be declared to accept and return the
polymorphic types described in [???](#extend-types-polymorphic), thus
allowing the actual data types handled by the function to vary from call
to call. Examples appear in [Declaring Function
Parameters](#plpgsql-declaration-parameters).

PL/pgSQL functions can also be declared to return a "set" (or table) of
any data type that can be returned as a single instance. Such a function
generates its output by executing `RETURN NEXT` for each desired element
of the result set, or by using `RETURN QUERY` to output the result of
evaluating a query.

Finally, a PL/pgSQL function can be declared to return `void` if it has
no useful return value. (Alternatively, it could be written as a
procedure in that case.)

PL/pgSQL functions can also be declared with output parameters in place
of an explicit specification of the return type. This does not add any
fundamental capability to the language, but it is often convenient,
especially for returning multiple values. The `RETURNS TABLE` notation
can also be used in place of `RETURNS SETOF`.

Specific examples appear in [Declaring Function
Parameters](#plpgsql-declaration-parameters) and [Returning from a
Function](#plpgsql-statements-returning).

Structure of PL/pgSQL {#plpgsql-structure}
=====================

Functions written in PL/pgSQL are defined to the server by executing
[???](#sql-createfunction) commands. Such a command would normally look
like, say,

    CREATE FUNCTION somefunc(integer, text) RETURNS integer
    AS 'function body text'
    LANGUAGE plpgsql;

The function body is simply a string literal so far as `CREATE
   FUNCTION` is concerned. It is often helpful to use dollar quoting
(see [???](#sql-syntax-dollar-quoting)) to write the function body,
rather than the normal single quote syntax. Without dollar quoting, any
single quotes or backslashes in the function body must be escaped by
doubling them. Almost all the examples in this chapter use dollar-quoted
literals for their function bodies.

PL/pgSQL is a block-structured language. The complete text of a function
body must be a block. A block is defined as: \[\<\<label\>\>\] \[DECLARE
declarations\] BEGIN statements END \[label\];

Each declaration and each statement within a block is terminated by a
semicolon. A block that appears within another block must have a
semicolon after `END`, as shown above; however the final `END` that
concludes a function body does not require a semicolon.

> **Tip**
>
> A common mistake is to write a semicolon immediately after `BEGIN`.
> This is incorrect and will result in a syntax error.

A label is only needed if you want to identify the block for use in an
`EXIT` statement, or to qualify the names of the variables declared in
the block. If a label is given after `END`, it must match the label at
the block\'s beginning.

All key words are case-insensitive. Identifiers are implicitly converted
to lower case unless double-quoted, just as they are in ordinary SQL
commands.

Comments work the same way in PL/pgSQL code as in ordinary SQL. A double
dash (`--`) starts a comment that extends to the end of the line. A `/*`
starts a block comment that extends to the matching occurrence of `*/`.
Block comments nest.

Any statement in the statement section of a block can be a subblock.
Subblocks can be used for logical grouping or to localize variables to a
small group of statements. Variables declared in a subblock mask any
similarly-named variables of outer blocks for the duration of the
subblock; but you can access the outer variables anyway if you qualify
their names with their block\'s label. For example:

    CREATE FUNCTION somefunc() RETURNS integer AS $$
    << outerblock >>
    DECLARE
        quantity integer := 30;
    BEGIN
        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 30
        quantity := 50;
        --
        -- Create a subblock
        --
        DECLARE
            quantity integer := 80;
        BEGIN
            RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 80
            RAISE NOTICE 'Outer quantity here is %', outerblock.quantity;  -- Prints 50
        END;

        RAISE NOTICE 'Quantity here is %', quantity;  -- Prints 50

        RETURN quantity;
    END;
    $$ LANGUAGE plpgsql;

> **Note**
>
> There is actually a hidden "outer block" surrounding the body of any
> PL/pgSQL function. This block provides the declarations of the
> function\'s parameters (if any), as well as some special variables
> such as `FOUND` (see [Obtaining the Result
> Status](#plpgsql-statements-diagnostics)). The outer block is labeled
> with the function\'s name, meaning that parameters and special
> variables can be qualified with the function\'s name.

It is important not to confuse the use of `BEGIN`/`END` for grouping
statements in PL/pgSQL with the similarly-named SQL commands for
transaction control. PL/pgSQL\'s `BEGIN`/`END` are only for grouping;
they do not start or end a transaction. See [Transaction
Management](#plpgsql-transactions) for information on managing
transactions in PL/pgSQL. Also, a block containing an `EXCEPTION` clause
effectively forms a subtransaction that can be rolled back without
affecting the outer transaction. For more about that see [Trapping
Errors](#plpgsql-error-trapping).

Declarations {#plpgsql-declarations}
============

All variables used in a block must be declared in the declarations
section of the block. (The only exceptions are that the loop variable of
a `FOR` loop iterating over a range of integer values is automatically
declared as an integer variable, and likewise the loop variable of a
`FOR` loop iterating over a cursor\'s result is automatically declared
as a record variable.)

PL/pgSQL variables can have any SQL data type, such as `integer`,
`varchar`, and `char`.

Here are some examples of variable declarations:

    user_id integer;
    quantity numeric(5);
    url varchar;
    myrow tablename%ROWTYPE;
    myfield tablename.columnname%TYPE;
    arow RECORD;

The general syntax of a variable declaration is: name \[CONSTANT\] type
\[COLLATE collation\_name\] \[NOT NULL\] \[{ DEFAULT \| := \| = }
expression\]; The `DEFAULT` clause, if given, specifies the initial
value assigned to the variable when the block is entered. If the
`DEFAULT` clause is not given then the variable is initialized to the
SQL null value. The `CONSTANT` option prevents the variable from being
assigned to after initialization, so that its value will remain constant
for the duration of the block. The `COLLATE` option specifies a
collation to use for the variable (see [Collation of
Variables](#plpgsql-declaration-collation)). If `NOT NULL` is specified,
an assignment of a null value results in a run-time error. All variables
declared as `NOT NULL` must have a nonnull default value specified.
Equal (`=`) can be used instead of PL/SQL-compliant `:=`.

A variable\'s default value is evaluated and assigned to the variable
each time the block is entered (not just once per function call). So,
for example, assigning `now()` to a variable of type `timestamp` causes
the variable to have the time of the current function call, not the time
when the function was precompiled.

Examples:

    quantity integer DEFAULT 32;
    url varchar := 'http://mysite.com';
    transaction_time CONSTANT timestamp with time zone := now();

Once declared, a variable\'s value can be used in later initialization
expressions in the same block, for example:

    DECLARE
      x integer := 1;
      y integer := x + 1;

Declaring Function Parameters {#plpgsql-declaration-parameters}
-----------------------------

Parameters passed to functions are named with the identifiers `$1`,
`$2`, etc. Optionally, aliases can be declared for `$n` parameter names
for increased readability. Either the alias or the numeric identifier
can then be used to refer to the parameter value.

There are two ways to create an alias. The preferred way is to give a
name to the parameter in the `CREATE FUNCTION` command, for example:

    CREATE FUNCTION sales_tax(subtotal real) RETURNS real AS $$
    BEGIN
        RETURN subtotal * 0.06;
    END;
    $$ LANGUAGE plpgsql;

The other way is to explicitly declare an alias, using the declaration
syntax name ALIAS FOR \$n; The same example in this style looks like:

    CREATE FUNCTION sales_tax(real) RETURNS real AS $$
    DECLARE
        subtotal ALIAS FOR $1;
    BEGIN
        RETURN subtotal * 0.06;
    END;
    $$ LANGUAGE plpgsql;

> **Note**
>
> These two examples are not perfectly equivalent. In the first case,
> `subtotal` could be referenced as `sales_tax.subtotal`, but in the
> second case it could not. (Had we attached a label to the inner block,
> `subtotal` could be qualified with that label, instead.)

Some more examples:

    CREATE FUNCTION instr(varchar, integer) RETURNS integer AS $$
    DECLARE
        v_string ALIAS FOR $1;
        index ALIAS FOR $2;
    BEGIN
        -- some computations using v_string and index here
    END;
    $$ LANGUAGE plpgsql;


    CREATE FUNCTION concat_selected_fields(in_t sometablename) RETURNS text AS $$
    BEGIN
        RETURN in_t.f1 || in_t.f3 || in_t.f5 || in_t.f7;
    END;
    $$ LANGUAGE plpgsql;

When a PL/pgSQL function is declared with output parameters, the output
parameters are given `$n` names and optional aliases in just the same
way as the normal input parameters. An output parameter is effectively a
variable that starts out NULL; it should be assigned to during the
execution of the function. The final value of the parameter is what is
returned. For instance, the sales-tax example could also be done this
way:

    CREATE FUNCTION sales_tax(subtotal real, OUT tax real) AS $$
    BEGIN
        tax := subtotal * 0.06;
    END;
    $$ LANGUAGE plpgsql;

Notice that we omitted `RETURNS real` MDASH we could have included it,
but it would be redundant.

To call a function with `OUT` parameters, omit the output parameter(s)
in the function call:

    SELECT sales_tax(100.00);

Output parameters are most useful when returning multiple values. A
trivial example is:

    CREATE FUNCTION sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
    BEGIN
        sum := x + y;
        prod := x * y;
    END;
    $$ LANGUAGE plpgsql;

    SELECT * FROM sum_n_product(2, 4);
     sum | prod
    -----+------
       6 |    8

As discussed in [???](#xfunc-output-parameters), this effectively
creates an anonymous record type for the function\'s results. If a
`RETURNS` clause is given, it must say `RETURNS record`.

This also works with procedures, for example:

    CREATE PROCEDURE sum_n_product(x int, y int, OUT sum int, OUT prod int) AS $$
    BEGIN
        sum := x + y;
        prod := x * y;
    END;
    $$ LANGUAGE plpgsql;

In a call to a procedure, all the parameters must be specified. For
output parameters, `NULL` may be specified when calling the procedure
from plain SQL:

    CALL sum_n_product(2, 4, NULL, NULL);
     sum | prod
    -----+------
       6 |    8

However, when calling a procedure from PL/pgSQL, you should instead
write a variable for any output parameter; the variable will receive the
result of the call. See [Calling a
Procedure](#plpgsql-statements-calling-procedure) for details.

Another way to declare a PL/pgSQL function is with `RETURNS TABLE`, for
example:

    CREATE FUNCTION extended_sales(p_itemno int)
    RETURNS TABLE(quantity int, total numeric) AS $$
    BEGIN
        RETURN QUERY SELECT s.quantity, s.quantity * s.price FROM sales AS s
                     WHERE s.itemno = p_itemno;
    END;
    $$ LANGUAGE plpgsql;

This is exactly equivalent to declaring one or more `OUT` parameters and
specifying `RETURNS SETOF
      sometype`.

When the return type of a PL/pgSQL function is declared as a polymorphic
type (see [???](#extend-types-polymorphic)), a special parameter `$0` is
created. Its data type is the actual return type of the function, as
deduced from the actual input types. This allows the function to access
its actual return type as shown in [Copying
Types](#plpgsql-declaration-type). `$0` is initialized to null and can
be modified by the function, so it can be used to hold the return value
if desired, though that is not required. `$0` can also be given an
alias. For example, this function works on any data type that has a `+`
operator:

    CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement)
    RETURNS anyelement AS $$
    DECLARE
        result ALIAS FOR $0;
    BEGIN
        result := v1 + v2 + v3;
        RETURN result;
    END;
    $$ LANGUAGE plpgsql;

The same effect can be obtained by declaring one or more output
parameters as polymorphic types. In this case the special `$0` parameter
is not used; the output parameters themselves serve the same purpose.
For example:

    CREATE FUNCTION add_three_values(v1 anyelement, v2 anyelement, v3 anyelement,
                                     OUT sum anyelement)
    AS $$
    BEGIN
        sum := v1 + v2 + v3;
    END;
    $$ LANGUAGE plpgsql;

In practice it might be more useful to declare a polymorphic function
using the `anycompatible` family of types, so that automatic promotion
of the input arguments to a common type will occur. For example:

    CREATE FUNCTION add_three_values(v1 anycompatible, v2 anycompatible, v3 anycompatible)
    RETURNS anycompatible AS $$
    BEGIN
        RETURN v1 + v2 + v3;
    END;
    $$ LANGUAGE plpgsql;

With this example, a call such as

    SELECT add_three_values(1, 2, 4.7);

will work, automatically promoting the integer inputs to numeric. The
function using `anyelement` would require you to cast the three inputs
to the same type manually.

`ALIAS` {#plpgsql-declaration-alias}
-------

newname

ALIAS FOR

oldname

;

The `ALIAS` syntax is more general than is suggested in the previous
section: you can declare an alias for any variable, not just function
parameters. The main practical use for this is to assign a different
name for variables with predetermined names, such as `NEW` or `OLD`
within a trigger function.

Examples:

    DECLARE
      prior ALIAS FOR old;
      updated ALIAS FOR new;

Since `ALIAS` creates two different ways to name the same object,
unrestricted use can be confusing. It\'s best to use it only for the
purpose of overriding predetermined names.

Copying Types {#plpgsql-declaration-type}
-------------

variable

\%TYPE

`%TYPE` provides the data type of a variable or table column. You can
use this to declare variables that will hold database values. For
example, let\'s say you have a column named `user_id` in your `users`
table. To declare a variable with the same data type as `users.user_id`
you write:

    user_id users.user_id%TYPE;

By using `%TYPE` you don\'t need to know the data type of the structure
you are referencing, and most importantly, if the data type of the
referenced item changes in the future (for instance: you change the type
of `user_id` from `integer` to `real`), you might not need to change
your function definition.

`%TYPE` is particularly valuable in polymorphic functions, since the
data types needed for internal variables can change from one call to the
next. Appropriate variables can be created by applying `%TYPE` to the
function\'s arguments or result placeholders.

Row Types {#plpgsql-declaration-rowtypes}
---------

name

table\_name

\%ROWTYPE

;

name

composite\_type\_name

;

A variable of a composite type is called a row variable (or row-type
variable). Such a variable can hold a whole row of a `SELECT` or `FOR`
query result, so long as that query\'s column set matches the declared
type of the variable. The individual fields of the row value are
accessed using the usual dot notation, for example `rowvar.field`.

A row variable can be declared to have the same type as the rows of an
existing table or view, by using the table\_name`%ROWTYPE` notation; or
it can be declared by giving a composite type\'s name. (Since every
table has an associated composite type of the same name, it actually
does not matter in PostgreSQL whether you write `%ROWTYPE` or not. But
the form with `%ROWTYPE` is more portable.)

Parameters to a function can be composite types (complete table rows).
In that case, the corresponding identifier `$n` will be a row variable,
and fields can be selected from it, for example `$1.user_id`.

Here is an example of using composite types. table1 and table2 are
existing tables having at least the mentioned fields:

    CREATE FUNCTION merge_fields(t_row table1) RETURNS text AS $$
    DECLARE
        t2_row table2%ROWTYPE;
    BEGIN
        SELECT * INTO t2_row FROM table2 WHERE ... ;
        RETURN t_row.f1 || t2_row.f3 || t_row.f5 || t2_row.f7;
    END;
    $$ LANGUAGE plpgsql;

    SELECT merge_fields(t.*) FROM table1 t WHERE ... ;

Record Types {#plpgsql-declaration-records}
------------

name

RECORD;

Record variables are similar to row-type variables, but they have no
predefined structure. They take on the actual row structure of the row
they are assigned during a `SELECT` or `FOR` command. The substructure
of a record variable can change each time it is assigned to. A
consequence of this is that until a record variable is first assigned
to, it has no substructure, and any attempt to access a field in it will
draw a run-time error.

Note that `RECORD` is not a true data type, only a placeholder. One
should also realize that when a PL/pgSQL function is declared to return
type `record`, this is not quite the same concept as a record variable,
even though such a function might use a record variable to hold its
result. In both cases the actual row structure is unknown when the
function is written, but for a function returning `record` the actual
structure is determined when the calling query is parsed, whereas a
record variable can change its row structure on-the-fly.

Collation of PL/pgSQL Variables {#plpgsql-declaration-collation}
-------------------------------

collation

in PL/pgSQL

When a PL/pgSQL function has one or more parameters of collatable data
types, a collation is identified for each function call depending on the
collations assigned to the actual arguments, as described in
[???](#collation). If a collation is successfully identified (i.e.,
there are no conflicts of implicit collations among the arguments) then
all the collatable parameters are treated as having that collation
implicitly. This will affect the behavior of collation-sensitive
operations within the function. For example, consider

    CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
    BEGIN
        RETURN a < b;
    END;
    $$ LANGUAGE plpgsql;

    SELECT less_than(text_field_1, text_field_2) FROM table1;
    SELECT less_than(text_field_1, text_field_2 COLLATE "C") FROM table1;

The first use of `less_than` will use the common collation of
text\_field\_1 and text\_field\_2 for the comparison, while the second
use will use `C` collation.

Furthermore, the identified collation is also assumed as the collation
of any local variables that are of collatable types. Thus this function
would not work any differently if it were written as

    CREATE FUNCTION less_than(a text, b text) RETURNS boolean AS $$
    DECLARE
        local_a text := a;
        local_b text := b;
    BEGIN
        RETURN local_a < local_b;
    END;
    $$ LANGUAGE plpgsql;

If there are no parameters of collatable data types, or no common
collation can be identified for them, then parameters and local
variables use the default collation of their data type (which is usually
the database\'s default collation, but could be different for variables
of domain types).

A local variable of a collatable data type can have a different
collation associated with it by including the `COLLATE` option in its
declaration, for example

    DECLARE
        local_a text COLLATE "en_US";

This option overrides the collation that would otherwise be given to the
variable according to the rules above.

Also, of course explicit `COLLATE` clauses can be written inside a
function if it is desired to force a particular collation to be used in
a particular operation. For example,

    CREATE FUNCTION less_than_c(a text, b text) RETURNS boolean AS $$
    BEGIN
        RETURN a < b COLLATE "C";
    END;
    $$ LANGUAGE plpgsql;

This overrides the collations associated with the table columns,
parameters, or local variables used in the expression, just as would
happen in a plain SQL command.

Expressions {#plpgsql-expressions}
===========

All expressions used in PL/pgSQL statements are processed using the
server\'s main SQL executor. For example, when you write a PL/pgSQL
statement like IF expression THEN \... PL/pgSQL will evaluate the
expression by feeding a query like SELECT expression to the main SQL
engine. While forming the `SELECT` command, any occurrences of PL/pgSQL
variable names are replaced by query parameters, as discussed in detail
in [Variable Substitution](#plpgsql-var-subst). This allows the query
plan for the `SELECT` to be prepared just once and then reused for
subsequent evaluations with different values of the variables. Thus,
what really happens on first use of an expression is essentially a
`PREPARE` command. For example, if we have declared two integer
variables `x` and `y`, and we write

    IF x < y THEN ...

what happens behind the scenes is equivalent to

    PREPARE statement_name(integer, integer) AS SELECT $1 < $2;

and then this prepared statement is `EXECUTE`d for each execution of the
`IF` statement, with the current values of the PL/pgSQL variables
supplied as parameter values. Normally these details are not important
to a PL/pgSQL user, but they are useful to know when trying to diagnose
a problem. More information appears in [Plan
Caching](#plpgsql-plan-caching).

Since an expression is converted to a `SELECT` command, it can contain
the same clauses that an ordinary `SELECT` would, except that it cannot
include a top-level `UNION`, `INTERSECT`, or `EXCEPT` clause. Thus for
example one could test whether a table is non-empty with

    IF count(*) > 0 FROM my_table THEN ...

since the expression between `IF` and `THEN` is parsed as though it were
`SELECT count(*) > 0 FROM my_table`. The `SELECT` must produce a single
column, and not more than one row. (If it produces no rows, the result
is taken as NULL.)

Basic Statements {#plpgsql-statements}
================

In this section and the following ones, we describe all the statement
types that are explicitly understood by PL/pgSQL. Anything not
recognized as one of these statement types is presumed to be an SQL
command and is sent to the main database engine to execute, as described
in [Executing SQL Commands](#plpgsql-statements-general-sql).

Assignment {#plpgsql-statements-assignment}
----------

An assignment of a value to a PL/pgSQL variable is written as: variable
{ := \| = } expression; As explained previously, the expression in such
a statement is evaluated by means of an SQL `SELECT` command sent to the
main database engine. The expression must yield a single value (possibly
a row value, if the variable is a row or record variable). The target
variable can be a simple variable (optionally qualified with a block
name), a field of a row or record target, or an element or slice of an
array target. Equal (`=`) can be used instead of PL/SQL-compliant `:=`.

If the expression\'s result data type doesn\'t match the variable\'s
data type, the value will be coerced as though by an assignment cast
(see [???](#typeconv-query)). If no assignment cast is known for the
pair of data types involved, the PL/pgSQL interpreter will attempt to
convert the result value textually, that is by applying the result
type\'s output function followed by the variable type\'s input function.
Note that this could result in run-time errors generated by the input
function, if the string form of the result value is not acceptable to
the input function.

Examples:

    tax := subtotal * 0.06;
    my_record.user_id := 20;
    my_array[j] := 20;
    my_array[1:3] := array[1,2,3];
    complex_array[n].realpart = 12.3;

Executing SQL Commands {#plpgsql-statements-general-sql}
----------------------

In general, any SQL command that does not return rows can be executed
within a PL/pgSQL function just by writing the command. For example, you
could create and fill a table by writing

    CREATE TABLE mytable (id int primary key, data text);
    INSERT INTO mytable VALUES (1,'one'), (2,'two');

If the command does return rows (for example `SELECT`, or
`INSERT`/`UPDATE`/`DELETE` with `RETURNING`), there are two ways to
proceed. When the command will return at most one row, or you only care
about the first row of output, write the command as usual but add an
`INTO` clause to capture the output, as described in [Executing a
Command with a Single-Row Result](#plpgsql-statements-sql-onerow). To
process all of the output rows, write the command as the data source for
a `FOR` loop, as described in [Looping through Query
Results](#plpgsql-records-iterating).

Usually it is not sufficient just to execute statically-defined SQL
commands. Typically you\'ll want a command to use varying data values,
or even to vary in more fundamental ways such as by using different
table names at different times. Again, there are two ways to proceed
depending on the situation.

PL/pgSQL variable values can be automatically inserted into optimizable
SQL commands, which are `SELECT`, `INSERT`, `UPDATE`, `DELETE`, and
certain utility commands that incorporate one of these, such as
`EXPLAIN` and `CREATE TABLE ... AS
     SELECT`. In these commands, any PL/pgSQL variable name appearing in
the command text is replaced by a query parameter, and then the current
value of the variable is provided as the parameter value at run time.
This is exactly like the processing described earlier for expressions;
for details see [Variable Substitution](#plpgsql-var-subst).

When executing an optimizable SQL command in this way, PL/pgSQL may
cache and re-use the execution plan for the command, as discussed in
[Plan Caching](#plpgsql-plan-caching).

Non-optimizable SQL commands (also called utility commands) are not
capable of accepting query parameters. So automatic substitution of
PL/pgSQL variables does not work in such commands. To include
non-constant text in a utility command executed from PL/pgSQL, you must
build the utility command as a string and then `EXECUTE` it, as
discussed in [Executing Dynamic
Commands](#plpgsql-statements-executing-dyn).

`EXECUTE` must also be used if you want to modify the command in some
other way than supplying a data value, for example by changing a table
name.

Sometimes it is useful to evaluate an expression or `SELECT` query but
discard the result, for example when calling a function that has
side-effects but no useful result value. To do this in PL/pgSQL, use the
`PERFORM` statement: PERFORM query; This executes query and discards the
result. Write the query the same way you would write an SQL `SELECT`
command, but replace the initial keyword `SELECT` with `PERFORM`. For
`WITH` queries, use `PERFORM` and then place the query in parentheses.
(In this case, the query can only return one row.) PL/pgSQL variables
will be substituted into the query just as described above, and the plan
is cached in the same way. Also, the special variable `FOUND` is set to
true if the query produced at least one row, or false if it produced no
rows (see [Obtaining the Result
Status](#plpgsql-statements-diagnostics)).

> **Note**
>
> One might expect that writing `SELECT` directly would accomplish this
> result, but at present the only accepted way to do it is `PERFORM`. An
> SQL command that can return rows, such as `SELECT`, will be rejected
> as an error unless it has an `INTO` clause as discussed in the next
> section.

An example:

    PERFORM create_mv('cs_session_page_requests_mv', my_query);

Executing a Command with a Single-Row Result {#plpgsql-statements-sql-onerow}
--------------------------------------------

SELECT INTO

in PL/pgSQL

RETURNING INTO

in PL/pgSQL

The result of an SQL command yielding a single row (possibly of multiple
columns) can be assigned to a record variable, row-type variable, or
list of scalar variables. This is done by writing the base SQL command
and adding an `INTO` clause. For example, SELECT select\_expressions
INTO \[STRICT\] target FROM \...; INSERT \... RETURNING expressions INTO
\[STRICT\] target; UPDATE \... RETURNING expressions INTO \[STRICT\]
target; DELETE \... RETURNING expressions INTO \[STRICT\] target; where
target can be a record variable, a row variable, or a comma-separated
list of simple variables and record/row fields. PL/pgSQL variables will
be substituted into the rest of the command (that is, everything but the
`INTO` clause) just as described above, and the plan is cached in the
same way. This works for `SELECT`, `INSERT`/`UPDATE`/`DELETE` with
`RETURNING`, and certain utility commands that return row sets, such as
`EXPLAIN`. Except for the `INTO` clause, the SQL command is the same as
it would be written outside PL/pgSQL.

> **Tip**
>
> Note that this interpretation of `SELECT` with `INTO` is quite
> different from PostgreSQL\'s regular `SELECT INTO` command, wherein
> the `INTO` target is a newly created table. If you want to create a
> table from a `SELECT` result inside a PL/pgSQL function, use the
> syntax `CREATE TABLE ... AS SELECT`.

If a row variable or a variable list is used as target, the command\'s
result columns must exactly match the structure of the target as to
number and data types, or else a run-time error occurs. When a record
variable is the target, it automatically configures itself to the row
type of the command\'s result columns.

The `INTO` clause can appear almost anywhere in the SQL command.
Customarily it is written either just before or just after the list of
select\_expressions in a `SELECT` command, or at the end of the command
for other command types. It is recommended that you follow this
convention in case the PL/pgSQL parser becomes stricter in future
versions.

If `STRICT` is not specified in the `INTO` clause, then target will be
set to the first row returned by the command, or to nulls if the command
returned no rows. (Note that "the first row" is not well-defined unless
you\'ve used `ORDER BY`.) Any result rows after the first row are
discarded. You can check the special `FOUND` variable (see [Obtaining
the Result Status](#plpgsql-statements-diagnostics)) to determine
whether a row was returned:

    SELECT * INTO myrec FROM emp WHERE empname = myname;
    IF NOT FOUND THEN
        RAISE EXCEPTION 'employee % not found', myname;
    END IF;

If the `STRICT` option is specified, the command must return exactly one
row or a run-time error will be reported, either `NO_DATA_FOUND` (no
rows) or `TOO_MANY_ROWS` (more than one row). You can use an exception
block if you wish to catch the error, for example:

    BEGIN
        SELECT * INTO STRICT myrec FROM emp WHERE empname = myname;
        EXCEPTION
            WHEN NO_DATA_FOUND THEN
                RAISE EXCEPTION 'employee % not found', myname;
            WHEN TOO_MANY_ROWS THEN
                RAISE EXCEPTION 'employee % not unique', myname;
    END;

Successful execution of a command with `STRICT` always sets `FOUND` to
true.

For `INSERT`/`UPDATE`/`DELETE` with `RETURNING`, PL/pgSQL reports an
error for more than one returned row, even when `STRICT` is not
specified. This is because there is no option such as `ORDER BY` with
which to determine which affected row should be returned.

If `print_strict_params` is enabled for the function, then when an error
is thrown because the requirements of `STRICT` are not met, the `DETAIL`
part of the error message will include information about the parameters
passed to the command. You can change the `print_strict_params` setting
for all functions by setting `plpgsql.print_strict_params`, though only
subsequent function compilations will be affected. You can also enable
it on a per-function basis by using a compiler option, for example:

    CREATE FUNCTION get_userid(username text) RETURNS int
    AS $$
    #print_strict_params on
    DECLARE
    userid int;
    BEGIN
        SELECT users.userid INTO STRICT userid
            FROM users WHERE users.username = get_userid.username;
        RETURN userid;
    END;
    $$ LANGUAGE plpgsql;

On failure, this function might produce an error message such as

    ERROR:  query returned no rows
    DETAIL:  parameters: $1 = 'nosuchuser'
    CONTEXT:  PL/pgSQL function get_userid(text) line 6 at SQL statement

> **Note**
>
> The `STRICT` option matches the behavior of Oracle PL/SQL\'s
> `SELECT INTO` and related statements.

Executing Dynamic Commands {#plpgsql-statements-executing-dyn}
--------------------------

Oftentimes you will want to generate dynamic commands inside your
PL/pgSQL functions, that is, commands that will involve different tables
or different data types each time they are executed. PL/pgSQL\'s normal
attempts to cache plans for commands (as discussed in [Plan
Caching](#plpgsql-plan-caching)) will not work in such scenarios. To
handle this sort of problem, the `EXECUTE` statement is provided:
EXECUTE command-string \[INTO \[STRICT\] target\] \[USING expression \[,
\...\]\]; where command-string is an expression yielding a string (of
type `text`) containing the command to be executed. The optional target
is a record variable, a row variable, or a comma-separated list of
simple variables and record/row fields, into which the results of the
command will be stored. The optional `USING` expressions supply values
to be inserted into the command.

No substitution of PL/pgSQL variables is done on the computed command
string. Any required variable values must be inserted in the command
string as it is constructed; or you can use parameters as described
below.

Also, there is no plan caching for commands executed via `EXECUTE`.
Instead, the command is always planned each time the statement is run.
Thus the command string can be dynamically created within the function
to perform actions on different tables and columns.

The `INTO` clause specifies where the results of an SQL command
returning rows should be assigned. If a row variable or variable list is
provided, it must exactly match the structure of the command\'s results;
if a record variable is provided, it will configure itself to match the
result structure automatically. If multiple rows are returned, only the
first will be assigned to the `INTO` variable(s). If no rows are
returned, NULL is assigned to the `INTO` variable(s). If no `INTO`
clause is specified, the command results are discarded.

If the `STRICT` option is given, an error is reported unless the command
produces exactly one row.

The command string can use parameter values, which are referenced in the
command as `$1`, `$2`, etc. These symbols refer to values supplied in
the `USING` clause. This method is often preferable to inserting data
values into the command string as text: it avoids run-time overhead of
converting the values to text and back, and it is much less prone to
SQL-injection attacks since there is no need for quoting or escaping. An
example is:

    EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
       INTO c
       USING checked_user, checked_date;

Note that parameter symbols can only be used for data values MDASH if
you want to use dynamically determined table or column names, you must
insert them into the command string textually. For example, if the
preceding query needed to be done against a dynamically selected table,
you could do this:

    EXECUTE 'SELECT count(*) FROM '
        || quote_ident(tabname)
        || ' WHERE inserted_by = $1 AND inserted <= $2'
       INTO c
       USING checked_user, checked_date;

A cleaner approach is to use `format()`\'s `%I` specification to insert
table or column names with automatic quoting:

    EXECUTE format('SELECT count(*) FROM %I '
       'WHERE inserted_by = $1 AND inserted <= $2', tabname)
       INTO c
       USING checked_user, checked_date;

(This example relies on the SQL rule that string literals separated by a
newline are implicitly concatenated.)

Another restriction on parameter symbols is that they only work in
optimizable SQL commands (`SELECT`, `INSERT`, `UPDATE`, `DELETE`,
`MERGE`, and certain commands containing one of these). In other
statement types (generically called utility statements), you must insert
values textually even if they are just data values.

An `EXECUTE` with a simple constant command string and some `USING`
parameters, as in the first example above, is functionally equivalent to
just writing the command directly in PL/pgSQL and allowing replacement
of PL/pgSQL variables to happen automatically. The important difference
is that `EXECUTE` will re-plan the command on each execution, generating
a plan that is specific to the current parameter values; whereas
PL/pgSQL may otherwise create a generic plan and cache it for re-use. In
situations where the best plan depends strongly on the parameter values,
it can be helpful to use `EXECUTE` to positively ensure that a generic
plan is not selected.

`SELECT INTO` is not currently supported within `EXECUTE`; instead,
execute a plain `SELECT` command and specify `INTO` as part of the
`EXECUTE` itself.

> **Note**
>
> The PL/pgSQL `EXECUTE` statement is not related to the
> [`EXECUTE`](#sql-execute) SQL statement supported by the PostgreSQL
> server. The server\'s `EXECUTE` statement cannot be used directly
> within PL/pgSQL functions (and is not needed).

quote\_ident

use in PL/pgSQL

quote\_literal

use in PL/pgSQL

quote\_nullable

use in PL/pgSQL

format

use in PL/pgSQL

When working with dynamic commands you will often have to handle
escaping of single quotes. The recommended method for quoting fixed text
in your function body is dollar quoting. (If you have legacy code that
does not use dollar quoting, please refer to the overview in [Handling
of Quotation Marks](#plpgsql-quote-tips), which can save you some effort
when translating said code to a more reasonable scheme.)

Dynamic values require careful handling since they might contain quote
characters. An example using `format()` (this assumes that you are
dollar quoting the function body so quote marks need not be doubled):

    EXECUTE format('UPDATE tbl SET %I = $1 '
       'WHERE key = $2', colname) USING newvalue, keyvalue;

It is also possible to call the quoting functions directly:

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = '
            || quote_literal(newvalue)
            || ' WHERE key = '
            || quote_literal(keyvalue);

This example demonstrates the use of the `quote_ident` and
`quote_literal` functions (see [???](#functions-string)). For safety,
expressions containing column or table identifiers should be passed
through `quote_ident` before insertion in a dynamic query. Expressions
containing values that should be literal strings in the constructed
command should be passed through `quote_literal`. These functions take
the appropriate steps to return the input text enclosed in double or
single quotes respectively, with any embedded special characters
properly escaped.

Because `quote_literal` is labeled `STRICT`, it will always return null
when called with a null argument. In the above example, if `newvalue` or
`keyvalue` were null, the entire dynamic query string would become null,
leading to an error from `EXECUTE`. You can avoid this problem by using
the `quote_nullable` function, which works the same as `quote_literal`
except that when called with a null argument it returns the string
`NULL`. For example,

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = '
            || quote_nullable(newvalue)
            || ' WHERE key = '
            || quote_nullable(keyvalue);

If you are dealing with values that might be null, you should usually
use `quote_nullable` in place of `quote_literal`.

As always, care must be taken to ensure that null values in a query do
not deliver unintended results. For example the `WHERE` clause

    'WHERE key = ' || quote_nullable(keyvalue)

will never succeed if `keyvalue` is null, because the result of using
the equality operator `=` with a null operand is always null. If you
wish null to work like an ordinary key value, you would need to rewrite
the above as

    'WHERE key IS NOT DISTINCT FROM ' || quote_nullable(keyvalue)

(At present, `IS NOT DISTINCT FROM` is handled much less efficiently
than `=`, so don\'t do this unless you must. See
[???](#functions-comparison) for more information on nulls and
`IS DISTINCT`.)

Note that dollar quoting is only useful for quoting fixed text. It would
be a very bad idea to try to write this example as:

    EXECUTE 'UPDATE tbl SET '
            || quote_ident(colname)
            || ' = $$'
            || newvalue
            || '$$ WHERE key = '
            || quote_literal(keyvalue);

because it would break if the contents of `newvalue` happened to contain
`$$`. The same objection would apply to any other dollar-quoting
delimiter you might pick. So, to safely quote text that is not known in
advance, you *must* use `quote_literal`, `quote_nullable`, or
`quote_ident`, as appropriate.

Dynamic SQL statements can also be safely constructed using the `format`
function (see [???](#functions-string-format)). For example:

    EXECUTE format('UPDATE tbl SET %I = %L '
       'WHERE key = %L', colname, newvalue, keyvalue);

`%I` is equivalent to `quote_ident`, and `%L` is equivalent to
`quote_nullable`. The `format` function can be used in conjunction with
the `USING` clause:

    EXECUTE format('UPDATE tbl SET %I = $1 WHERE key = $2', colname)
       USING newvalue, keyvalue;

This form is better because the variables are handled in their native
data type format, rather than unconditionally converting them to text
and quoting them via `%L`. It is also more efficient.

A much larger example of a dynamic command and `EXECUTE` can be seen in
[example\_title](#plpgsql-porting-ex2), which builds and executes a
`CREATE FUNCTION` command to define a new function.

Obtaining the Result Status {#plpgsql-statements-diagnostics}
---------------------------

There are several ways to determine the effect of a command. The first
method is to use the `GET DIAGNOSTICS` command, which has the form: GET
\[CURRENT\] DIAGNOSTICS variable { = \| := } item \[, \...\]; This
command allows retrieval of system status indicators. `CURRENT` is a
noise word (but see also `GET STACKED
     DIAGNOSTICS` in [Obtaining Information about an
Error](#plpgsql-exception-diagnostics)). Each item is a key word
identifying a status value to be assigned to the specified variable
(which should be of the right data type to receive it). The currently
available status items are shown in
[table\_title](#plpgsql-current-diagnostics-values). Colon-equal (`:=`)
can be used instead of the SQL-standard `=` token. An example:

    GET DIAGNOSTICS integer_var = ROW_COUNT;

  --------------------------------------------------------------------------
  Name               Type               Description
  ------------------ ------------------ ------------------------------------
  `ROW_COUNT`        `bigint`           the number of rows processed by the
                                        most recent SQL command

  `PG_CONTEXT`       `text`             line(s) of text describing the
                                        current call stack (see [Obtaining
                                        Execution Location
                                        Information](#plpgsql-call-stack))
  --------------------------------------------------------------------------

  : Available Diagnostics Items

The second method to determine the effects of a command is to check the
special variable named `FOUND`, which is of type `boolean`. `FOUND`
starts out false within each PL/pgSQL function call. It is set by each
of the following types of statements:

-   A `SELECT INTO` statement sets `FOUND` true if a row is assigned,
    false if no row is returned.

-   A `PERFORM` statement sets `FOUND` true if it produces (and
    discards) one or more rows, false if no row is produced.

-   `UPDATE`, `INSERT`, `DELETE`, and `MERGE` statements set `FOUND`
    true if at least one row is affected, false if no row is affected.

-   A `FETCH` statement sets `FOUND` true if it returns a row, false if
    no row is returned.

-   A `MOVE` statement sets `FOUND` true if it successfully repositions
    the cursor, false otherwise.

-   A `FOR` or `FOREACH` statement sets `FOUND` true if it iterates one
    or more times, else false. `FOUND` is set this way when the loop
    exits; inside the execution of the loop, `FOUND` is not modified by
    the loop statement, although it might be changed by the execution of
    other statements within the loop body.

-   `RETURN QUERY` and `RETURN QUERY
                EXECUTE` statements set `FOUND` true if the query
    returns at least one row, false if no row is returned.

Other PL/pgSQL statements do not change the state of `FOUND`. Note in
particular that `EXECUTE` changes the output of `GET DIAGNOSTICS`, but
does not change `FOUND`.

`FOUND` is a local variable within each PL/pgSQL function; any changes
to it affect only the current function.

Doing Nothing At All {#plpgsql-statements-null}
--------------------

Sometimes a placeholder statement that does nothing is useful. For
example, it can indicate that one arm of an if/then/else chain is
deliberately empty. For this purpose, use the `NULL` statement: NULL;

For example, the following two fragments of code are equivalent:

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            NULL;  -- ignore the error
    END;

    BEGIN
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN  -- ignore the error
    END;

Which is preferable is a matter of taste.

> **Note**
>
> In Oracle\'s PL/SQL, empty statement lists are not allowed, and so
> `NULL` statements are *required* for situations such as this. PL/pgSQL
> allows you to just write nothing, instead.

Control Structures {#plpgsql-control-structures}
==================

Control structures are probably the most useful (and important) part of
PL/pgSQL. With PL/pgSQL\'s control structures, you can manipulate
PostgreSQL data in a very flexible and powerful way.

Returning from a Function {#plpgsql-statements-returning}
-------------------------

There are two commands available that allow you to return data from a
function: `RETURN` and `RETURN
     NEXT`.

### `RETURN`

RETURN

expression

;

`RETURN` with an expression terminates the function and returns the
value of expression to the caller. This form is used for PL/pgSQL
functions that do not return a set.

In a function that returns a scalar type, the expression\'s result will
automatically be cast into the function\'s return type as described for
assignments. But to return a composite (row) value, you must write an
expression delivering exactly the requested column set. This may require
use of explicit casting.

If you declared the function with output parameters, write just `RETURN`
with no expression. The current values of the output parameter variables
will be returned.

If you declared the function to return `void`, a `RETURN` statement can
be used to exit the function early; but do not write an expression
following `RETURN`.

The return value of a function cannot be left undefined. If control
reaches the end of the top-level block of the function without hitting a
`RETURN` statement, a run-time error will occur. This restriction does
not apply to functions with output parameters and functions returning
`void`, however. In those cases a `RETURN` statement is automatically
executed if the top-level block finishes.

Some examples:

    -- functions returning a scalar type
    RETURN 1 + 2;
    RETURN scalar_var;

    -- functions returning a composite type
    RETURN composite_type_var;
    RETURN (1, 2, 'three'::text);  -- must cast columns to correct types

### `RETURN NEXT` and `RETURN QUERY`

RETURN NEXT

in PL/pgSQL

RETURN QUERY

in PL/pgSQL

RETURN NEXT

expression

; RETURN QUERY

query

; RETURN QUERY EXECUTE

command-string

USING

expression

, \...

;

When a PL/pgSQL function is declared to return `SETOF sometype`, the
procedure to follow is slightly different. In that case, the individual
items to return are specified by a sequence of `RETURN
      NEXT` or `RETURN QUERY` commands, and then a final `RETURN`
command with no argument is used to indicate that the function has
finished executing. `RETURN NEXT` can be used with both scalar and
composite data types; with a composite result type, an entire "table" of
results will be returned. `RETURN QUERY` appends the results of
executing a query to the function\'s result set. `RETURN
      NEXT` and `RETURN QUERY` can be freely intermixed in a single
set-returning function, in which case their results will be
concatenated.

`RETURN NEXT` and `RETURN
      QUERY` do not actually return from the function MDASH they simply
append zero or more rows to the function\'s result set. Execution then
continues with the next statement in the PL/pgSQL function. As
successive `RETURN NEXT` or `RETURN
      QUERY` commands are executed, the result set is built up. A final
`RETURN`, which should have no argument, causes control to exit the
function (or you can just let control reach the end of the function).

`RETURN QUERY` has a variant `RETURN QUERY EXECUTE`, which specifies the
query to be executed dynamically. Parameter expressions can be inserted
into the computed query string via `USING`, in just the same way as in
the `EXECUTE` command.

If you declared the function with output parameters, write just
`RETURN NEXT` with no expression. On each execution, the current values
of the output parameter variable(s) will be saved for eventual return as
a row of the result. Note that you must declare the function as
returning `SETOF record` when there are multiple output parameters, or
`SETOF sometype` when there is just one output parameter of type
sometype, in order to create a set-returning function with output
parameters.

Here is an example of a function using `RETURN
      NEXT`:

    CREATE TABLE foo (fooid INT, foosubid INT, fooname TEXT);
    INSERT INTO foo VALUES (1, 2, 'three');
    INSERT INTO foo VALUES (4, 5, 'six');

    CREATE OR REPLACE FUNCTION get_all_foo() RETURNS SETOF foo AS
    $BODY$
    DECLARE
        r foo%rowtype;
    BEGIN
        FOR r IN
            SELECT * FROM foo WHERE fooid > 0
        LOOP
            -- can do some processing here
            RETURN NEXT r; -- return current row of SELECT
        END LOOP;
        RETURN;
    END;
    $BODY$
    LANGUAGE plpgsql;

    SELECT * FROM get_all_foo();

Here is an example of a function using `RETURN
      QUERY`:

    CREATE FUNCTION get_available_flightid(date) RETURNS SETOF integer AS
    $BODY$
    BEGIN
        RETURN QUERY SELECT flightid
                       FROM flight
                      WHERE flightdate >= $1
                        AND flightdate < ($1 + 1);

        -- Since execution is not finished, we can check whether rows were returned
        -- and raise exception if not.
        IF NOT FOUND THEN
            RAISE EXCEPTION 'No flight at %.', $1;
        END IF;

        RETURN;
     END;
    $BODY$
    LANGUAGE plpgsql;

    -- Returns available flights or raises exception if there are no
    -- available flights.
    SELECT * FROM get_available_flightid(CURRENT_DATE);

> **Note**
>
> The current implementation of `RETURN NEXT` and `RETURN QUERY` stores
> the entire result set before returning from the function, as discussed
> above. That means that if a PL/pgSQL function produces a very large
> result set, performance might be poor: data will be written to disk to
> avoid memory exhaustion, but the function itself will not return until
> the entire result set has been generated. A future version of PL/pgSQL
> might allow users to define set-returning functions that do not have
> this limitation. Currently, the point at which data begins being
> written to disk is controlled by the [???](#guc-work-mem)
> configuration variable. Administrators who have sufficient memory to
> store larger result sets in memory should consider increasing this
> parameter.

Returning from a Procedure {#plpgsql-statements-returning-procedure}
--------------------------

A procedure does not have a return value. A procedure can therefore end
without a `RETURN` statement. If you wish to use a `RETURN` statement to
exit the code early, write just `RETURN` with no expression.

If the procedure has output parameters, the final values of the output
parameter variables will be returned to the caller.

Calling a Procedure {#plpgsql-statements-calling-procedure}
-------------------

A PL/pgSQL function, procedure, or `DO` block can call a procedure using
`CALL`. Output parameters are handled differently from the way that
`CALL` works in plain SQL. Each `OUT` or `INOUT` parameter of the
procedure must correspond to a variable in the `CALL` statement, and
whatever the procedure returns is assigned back to that variable after
it returns. For example:

    CREATE PROCEDURE triple(INOUT x int)
    LANGUAGE plpgsql
    AS $$
    BEGIN
        x := x * 3;
    END;
    $$;

    DO $$
    DECLARE myvar int := 5;
    BEGIN
      CALL triple(myvar);
      RAISE NOTICE 'myvar = %', myvar;  -- prints 15
    END;
    $$;

The variable corresponding to an output parameter can be a simple
variable or a field of a composite-type variable. Currently, it cannot
be an element of an array.

Conditionals {#plpgsql-conditionals}
------------

`IF` and `CASE` statements let you execute alternative commands based on
certain conditions. PL/pgSQL has three forms of `IF`:

-   `IF ... THEN ... END IF`

-   `IF ... THEN ... ELSE ... END IF`

-   `IF ... THEN ... ELSIF ... THEN ... ELSE ... END IF`

and two forms of `CASE`:

-   `CASE ... WHEN ... THEN ... ELSE ... END CASE`

-   `CASE WHEN ... THEN ... ELSE ... END CASE`

### `IF-THEN`

IF

boolean-expression

THEN

statements

END IF;

`IF-THEN` statements are the simplest form of `IF`. The statements
between `THEN` and `END IF` will be executed if the condition is true.
Otherwise, they are skipped.

Example:

    IF v_user_id <> 0 THEN
        UPDATE users SET email = v_email WHERE user_id = v_user_id;
    END IF;

### `IF-THEN-ELSE`

IF

boolean-expression

THEN

statements

ELSE

statements

END IF;

`IF-THEN-ELSE` statements add to `IF-THEN` by letting you specify an
alternative set of statements that should be executed if the condition
is not true. (Note this includes the case where the condition evaluates
to NULL.)

Examples:

    IF parentid IS NULL OR parentid = ''
    THEN
        RETURN fullname;
    ELSE
        RETURN hp_true_filename(parentid) || '/' || fullname;
    END IF;

    IF v_count > 0 THEN
        INSERT INTO users_count (count) VALUES (v_count);
        RETURN 't';
    ELSE
        RETURN 'f';
    END IF;

### `IF-THEN-ELSIF`

IF

boolean-expression

THEN

statements

ELSIF

boolean-expression

THEN

statements

ELSIF

boolean-expression

THEN

statements

\...

ELSE

statements

END IF;

Sometimes there are more than just two alternatives. `IF-THEN-ELSIF`
provides a convenient method of checking several alternatives in turn.
The `IF` conditions are tested successively until the first one that is
true is found. Then the associated statement(s) are executed, after
which control passes to the next statement after `END IF`. (Any
subsequent `IF` conditions are *not* tested.) If none of the `IF`
conditions is true, then the `ELSE` block (if any) is executed.

Here is an example:

    IF number = 0 THEN
        result := 'zero';
    ELSIF number > 0 THEN
        result := 'positive';
    ELSIF number < 0 THEN
        result := 'negative';
    ELSE
        -- hmm, the only other possibility is that number is null
        result := 'NULL';
    END IF;

The key word `ELSIF` can also be spelled `ELSEIF`.

An alternative way of accomplishing the same task is to nest
`IF-THEN-ELSE` statements, as in the following example:

    IF demo_row.sex = 'm' THEN
        pretty_sex := 'man';
    ELSE
        IF demo_row.sex = 'f' THEN
            pretty_sex := 'woman';
        END IF;
    END IF;

However, this method requires writing a matching `END IF` for each `IF`,
so it is much more cumbersome than using `ELSIF` when there are many
alternatives.

### Simple `CASE`

CASE

search-expression

WHEN

expression

,

expression

\...

THEN

statements

WHEN

expression

,

expression

\...

THEN

statements

\...

ELSE

statements

END CASE;

The simple form of `CASE` provides conditional execution based on
equality of operands. The search-expression is evaluated (once) and
successively compared to each expression in the `WHEN` clauses. If a
match is found, then the corresponding statements are executed, and then
control passes to the next statement after `END CASE`. (Subsequent
`WHEN` expressions are not evaluated.) If no match is found, the `ELSE`
statements are executed; but if `ELSE` is not present, then a
`CASE_NOT_FOUND` exception is raised.

Here is a simple example:

    CASE x
        WHEN 1, 2 THEN
            msg := 'one or two';
        ELSE
            msg := 'other value than one or two';
    END CASE;

### Searched `CASE`

CASE WHEN

boolean-expression

THEN

statements

WHEN

boolean-expression

THEN

statements

\...

ELSE

statements

END CASE;

The searched form of `CASE` provides conditional execution based on
truth of Boolean expressions. Each `WHEN` clause\'s boolean-expression
is evaluated in turn, until one is found that yields `true`. Then the
corresponding statements are executed, and then control passes to the
next statement after `END CASE`. (Subsequent `WHEN` expressions are not
evaluated.) If no true result is found, the `ELSE` statements are
executed; but if `ELSE` is not present, then a `CASE_NOT_FOUND`
exception is raised.

Here is an example:

    CASE
        WHEN x BETWEEN 0 AND 10 THEN
            msg := 'value is between zero and ten';
        WHEN x BETWEEN 11 AND 20 THEN
            msg := 'value is between eleven and twenty';
    END CASE;

This form of `CASE` is entirely equivalent to `IF-THEN-ELSIF`, except
for the rule that reaching an omitted `ELSE` clause results in an error
rather than doing nothing.

Simple Loops {#plpgsql-control-structures-loops}
------------

loop

in PL/pgSQL

With the `LOOP`, `EXIT`, `CONTINUE`, `WHILE`, `FOR`, and `FOREACH`
statements, you can arrange for your PL/pgSQL function to repeat a
series of commands.

### `LOOP`

\<\<

label

\>\>

LOOP

statements

END LOOP

label

;

`LOOP` defines an unconditional loop that is repeated indefinitely until
terminated by an `EXIT` or `RETURN` statement. The optional label can be
used by `EXIT` and `CONTINUE` statements within nested loops to specify
which loop those statements refer to.

### `EXIT`

EXIT

in PL/pgSQL

EXIT

label

WHEN

boolean-expression

;

If no label is given, the innermost loop is terminated and the statement
following `END
        LOOP` is executed next. If label is given, it must be the label
of the current or some outer level of nested loop or block. Then the
named loop or block is terminated and control continues with the
statement after the loop\'s/block\'s corresponding `END`.

If `WHEN` is specified, the loop exit occurs only if boolean-expression
is true. Otherwise, control passes to the statement after `EXIT`.

`EXIT` can be used with all types of loops; it is not limited to use
with unconditional loops.

When used with a `BEGIN` block, `EXIT` passes control to the next
statement after the end of the block. Note that a label must be used for
this purpose; an unlabeled `EXIT` is never considered to match a `BEGIN`
block. (This is a change from pre-8.4 releases of PostgreSQL, which
would allow an unlabeled `EXIT` to match a `BEGIN` block.)

Examples:

    LOOP
        -- some computations
        IF count > 0 THEN
            EXIT;  -- exit loop
        END IF;
    END LOOP;

    LOOP
        -- some computations
        EXIT WHEN count > 0;  -- same result as previous example
    END LOOP;

    <<ablock>>
    BEGIN
        -- some computations
        IF stocks > 100000 THEN
            EXIT ablock;  -- causes exit from the BEGIN block
        END IF;
        -- computations here will be skipped when stocks > 100000
    END;

### `CONTINUE`

CONTINUE

in PL/pgSQL

CONTINUE

label

WHEN

boolean-expression

;

If no label is given, the next iteration of the innermost loop is begun.
That is, all statements remaining in the loop body are skipped, and
control returns to the loop control expression (if any) to determine
whether another loop iteration is needed. If label is present, it
specifies the label of the loop whose execution will be continued.

If `WHEN` is specified, the next iteration of the loop is begun only if
boolean-expression is true. Otherwise, control passes to the statement
after `CONTINUE`.

`CONTINUE` can be used with all types of loops; it is not limited to use
with unconditional loops.

Examples:

    LOOP
        -- some computations
        EXIT WHEN count > 100;
        CONTINUE WHEN count < 50;
        -- some computations for count IN [50 .. 100]
    END LOOP;

### `WHILE`

WHILE

in PL/pgSQL

\<\<

label

\>\>

WHILE

boolean-expression

LOOP

statements

END LOOP

label

;

The `WHILE` statement repeats a sequence of statements so long as the
boolean-expression evaluates to true. The expression is checked just
before each entry to the loop body.

For example:

    WHILE amount_owed > 0 AND gift_certificate_balance > 0 LOOP
        -- some computations here
    END LOOP;

    WHILE NOT done LOOP
        -- some computations here
    END LOOP;

### `FOR` (Integer Variant) {#plpgsql-integer-for}

\<\<

label

\>\>

FOR

name

IN

REVERSE

expression

..

expression

BY

expression

LOOP

statements

END LOOP

label

;

This form of `FOR` creates a loop that iterates over a range of integer
values. The variable name is automatically defined as type `integer` and
exists only inside the loop (any existing definition of the variable
name is ignored within the loop). The two expressions giving the lower
and upper bound of the range are evaluated once when entering the loop.
If the `BY` clause isn\'t specified the iteration step is 1, otherwise
it\'s the value specified in the `BY` clause, which again is evaluated
once on loop entry. If `REVERSE` is specified then the step value is
subtracted, rather than added, after each iteration.

Some examples of integer `FOR` loops:

    FOR i IN 1..10 LOOP
        -- i will take on the values 1,2,3,4,5,6,7,8,9,10 within the loop
    END LOOP;

    FOR i IN REVERSE 10..1 LOOP
        -- i will take on the values 10,9,8,7,6,5,4,3,2,1 within the loop
    END LOOP;

    FOR i IN REVERSE 10..1 BY 2 LOOP
        -- i will take on the values 10,8,6,4,2 within the loop
    END LOOP;

If the lower bound is greater than the upper bound (or less than, in the
`REVERSE` case), the loop body is not executed at all. No error is
raised.

If a label is attached to the `FOR` loop then the integer loop variable
can be referenced with a qualified name, using that label.

Looping through Query Results {#plpgsql-records-iterating}
-----------------------------

Using a different type of `FOR` loop, you can iterate through the
results of a query and manipulate that data accordingly. The syntax is:
\[\<\<label\>\>\] FOR target IN query LOOP statements END LOOP
\[label\]; The target is a record variable, row variable, or
comma-separated list of scalar variables. The target is successively
assigned each row resulting from the query and the loop body is executed
for each row. Here is an example:

    CREATE FUNCTION refresh_mviews() RETURNS integer AS $$
    DECLARE
        mviews RECORD;
    BEGIN
        RAISE NOTICE 'Refreshing all materialized views...';

        FOR mviews IN
           SELECT n.nspname AS mv_schema,
                  c.relname AS mv_name,
                  pg_catalog.pg_get_userbyid(c.relowner) AS owner
             FROM pg_catalog.pg_class c
        LEFT JOIN pg_catalog.pg_namespace n ON (n.oid = c.relnamespace)
            WHERE c.relkind = 'm'
         ORDER BY 1
        LOOP

            -- Now "mviews" has one record with information about the materialized view

            RAISE NOTICE 'Refreshing materialized view %.% (owner: %)...',
                         quote_ident(mviews.mv_schema),
                         quote_ident(mviews.mv_name),
                         quote_ident(mviews.owner);
            EXECUTE format('REFRESH MATERIALIZED VIEW %I.%I', mviews.mv_schema, mviews.mv_name);
        END LOOP;

        RAISE NOTICE 'Done refreshing materialized views.';
        RETURN 1;
    END;
    $$ LANGUAGE plpgsql;

If the loop is terminated by an `EXIT` statement, the last assigned row
value is still accessible after the loop.

The query used in this type of `FOR` statement can be any SQL command
that returns rows to the caller: `SELECT` is the most common case, but
you can also use `INSERT`, `UPDATE`, or `DELETE` with a `RETURNING`
clause. Some utility commands such as `EXPLAIN` will work too.

PL/pgSQL variables are replaced by query parameters, and the query plan
is cached for possible re-use, as discussed in detail in [Variable
Substitution](#plpgsql-var-subst) and [Plan
Caching](#plpgsql-plan-caching).

The `FOR-IN-EXECUTE` statement is another way to iterate over rows:
\[\<\<label\>\>\] FOR target IN EXECUTE text\_expression \[USING
expression \[, \...\]\] LOOP statements END LOOP \[label\]; This is like
the previous form, except that the source query is specified as a string
expression, which is evaluated and replanned on each entry to the `FOR`
loop. This allows the programmer to choose the speed of a preplanned
query or the flexibility of a dynamic query, just as with a plain
`EXECUTE` statement. As with `EXECUTE`, parameter values can be inserted
into the dynamic command via `USING`.

Another way to specify the query whose results should be iterated
through is to declare it as a cursor. This is described in [Looping
through a Cursor\'s Result](#plpgsql-cursor-for-loop).

Looping through Arrays {#plpgsql-foreach-array}
----------------------

The `FOREACH` loop is much like a `FOR` loop, but instead of iterating
through the rows returned by an SQL query, it iterates through the
elements of an array value. (In general, `FOREACH` is meant for looping
through components of a composite-valued expression; variants for
looping through composites besides arrays may be added in future.) The
`FOREACH` statement to loop over an array is: \[\<\<label\>\>\] FOREACH
target \[SLICE number\] IN ARRAY expression LOOP statements END LOOP
\[label\];

Without `SLICE`, or if `SLICE 0` is specified, the loop iterates through
individual elements of the array produced by evaluating the expression.
The target variable is assigned each element value in sequence, and the
loop body is executed for each element. Here is an example of looping
through the elements of an integer array:

    CREATE FUNCTION sum(int[]) RETURNS int8 AS $$
    DECLARE
      s int8 := 0;
      x int;
    BEGIN
      FOREACH x IN ARRAY $1
      LOOP
        s := s + x;
      END LOOP;
      RETURN s;
    END;
    $$ LANGUAGE plpgsql;

The elements are visited in storage order, regardless of the number of
array dimensions. Although the target is usually just a single variable,
it can be a list of variables when looping through an array of composite
values (records). In that case, for each array element, the variables
are assigned from successive columns of the composite value.

With a positive `SLICE` value, `FOREACH` iterates through slices of the
array rather than single elements. The `SLICE` value must be an integer
constant not larger than the number of dimensions of the array. The
target variable must be an array, and it receives successive slices of
the array value, where each slice is of the number of dimensions
specified by `SLICE`. Here is an example of iterating through
one-dimensional slices:

    CREATE FUNCTION scan_rows(int[]) RETURNS void AS $$
    DECLARE
      x int[];
    BEGIN
      FOREACH x SLICE 1 IN ARRAY $1
      LOOP
        RAISE NOTICE 'row = %', x;
      END LOOP;
    END;
    $$ LANGUAGE plpgsql;

    SELECT scan_rows(ARRAY[[1,2,3],[4,5,6],[7,8,9],[10,11,12]]);

    NOTICE:  row = {1,2,3}
    NOTICE:  row = {4,5,6}
    NOTICE:  row = {7,8,9}
    NOTICE:  row = {10,11,12}

Trapping Errors {#plpgsql-error-trapping}
---------------

exceptions

in PL/pgSQL

By default, any error occurring in a PL/pgSQL function aborts execution
of the function and the surrounding transaction. You can trap errors and
recover from them by using a `BEGIN` block with an `EXCEPTION` clause.
The syntax is an extension of the normal syntax for a `BEGIN` block:
\[\<\<label\>\>\] \[DECLARE declarations\] BEGIN statements EXCEPTION
WHEN condition \[OR condition \...\] THEN handler\_statements \[WHEN
condition \[OR condition \...\] THEN handler\_statements \...\] END;

If no error occurs, this form of block simply executes all the
statements, and then control passes to the next statement after `END`.
But if an error occurs within the statements, further processing of the
statements is abandoned, and control passes to the `EXCEPTION` list. The
list is searched for the first condition matching the error that
occurred. If a match is found, the corresponding handler\_statements are
executed, and then control passes to the next statement after `END`. If
no match is found, the error propagates out as though the `EXCEPTION`
clause were not there at all: the error can be caught by an enclosing
block with `EXCEPTION`, or if there is none it aborts processing of the
function.

The condition names can be any of those shown in
[???](#errcodes-appendix). A category name matches any error within its
category. The special condition name `OTHERS` matches every error type
except `QUERY_CANCELED` and `ASSERT_FAILURE`. (It is possible, but often
unwise, to trap those two error types by name.) Condition names are not
case-sensitive. Also, an error condition can be specified by `SQLSTATE`
code; for example these are equivalent:

    WHEN division_by_zero THEN ...
    WHEN SQLSTATE '22012' THEN ...

If a new error occurs within the selected handler\_statements, it cannot
be caught by this `EXCEPTION` clause, but is propagated out. A
surrounding `EXCEPTION` clause could catch it.

When an error is caught by an `EXCEPTION` clause, the local variables of
the PL/pgSQL function remain as they were when the error occurred, but
all changes to persistent database state within the block are rolled
back. As an example, consider this fragment:

    INSERT INTO mytab(firstname, lastname) VALUES('Tom', 'Jones');
    BEGIN
        UPDATE mytab SET firstname = 'Joe' WHERE lastname = 'Jones';
        x := x + 1;
        y := x / 0;
    EXCEPTION
        WHEN division_by_zero THEN
            RAISE NOTICE 'caught division_by_zero';
            RETURN x;
    END;

When control reaches the assignment to `y`, it will fail with a
`division_by_zero` error. This will be caught by the `EXCEPTION` clause.
The value returned in the `RETURN` statement will be the incremented
value of `x`, but the effects of the `UPDATE` command will have been
rolled back. The `INSERT` command preceding the block is not rolled
back, however, so the end result is that the database contains
`Tom Jones` not `Joe Jones`.

> **Tip**
>
> A block containing an `EXCEPTION` clause is significantly more
> expensive to enter and exit than a block without one. Therefore,
> don\'t use `EXCEPTION` without need.

This example uses exception handling to perform either `UPDATE` or
`INSERT`, as appropriate. It is recommended that applications use
`INSERT` with `ON CONFLICT DO UPDATE` rather than actually using this
pattern. This example serves primarily to illustrate use of PL/pgSQL
control flow structures:

    CREATE TABLE db (a INT PRIMARY KEY, b TEXT);

    CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
    $$
    BEGIN
        LOOP
            -- first try to update the key
            UPDATE db SET b = data WHERE a = key;
            IF found THEN
                RETURN;
            END IF;
            -- not there, so try to insert the key
            -- if someone else inserts the same key concurrently,
            -- we could get a unique-key failure
            BEGIN
                INSERT INTO db(a,b) VALUES (key, data);
                RETURN;
            EXCEPTION WHEN unique_violation THEN
                -- Do nothing, and loop to try the UPDATE again.
            END;
        END LOOP;
    END;
    $$
    LANGUAGE plpgsql;

    SELECT merge_db(1, 'david');
    SELECT merge_db(1, 'dennis');

This coding assumes the `unique_violation` error is caused by the
`INSERT`, and not by, say, an `INSERT` in a trigger function on the
table. It might also misbehave if there is more than one unique index on
the table, since it will retry the operation regardless of which index
caused the error. More safety could be had by using the features
discussed next to check that the trapped error was the one expected.

### Obtaining Information about an Error {#plpgsql-exception-diagnostics}

Exception handlers frequently need to identify the specific error that
occurred. There are two ways to get information about the current
exception in PL/pgSQL: special variables and the
`GET STACKED DIAGNOSTICS` command.

Within an exception handler, the special variable `SQLSTATE` contains
the error code that corresponds to the exception that was raised (refer
to [???](#errcodes-table) for a list of possible error codes). The
special variable `SQLERRM` contains the error message associated with
the exception. These variables are undefined outside exception handlers.

Within an exception handler, one may also retrieve information about the
current exception by using the `GET STACKED DIAGNOSTICS` command, which
has the form: GET STACKED DIAGNOSTICS variable { = \| := } item \[,
\...\]; Each item is a key word identifying a status value to be
assigned to the specified variable (which should be of the right data
type to receive it). The currently available status items are shown in
[table\_title](#plpgsql-exception-diagnostics-values).

  --------------------------------------------------------------------------------
  Name                         Type           Description
  ---------------------------- -------------- ------------------------------------
  `RETURNED_SQLSTATE`          `text`         the SQLSTATE error code of the
                                              exception

  `COLUMN_NAME`                `text`         the name of the column related to
                                              exception

  `CONSTRAINT_NAME`            `text`         the name of the constraint related
                                              to exception

  `PG_DATATYPE_NAME`           `text`         the name of the data type related to
                                              exception

  `MESSAGE_TEXT`               `text`         the text of the exception\'s primary
                                              message

  `TABLE_NAME`                 `text`         the name of the table related to
                                              exception

  `SCHEMA_NAME`                `text`         the name of the schema related to
                                              exception

  `PG_EXCEPTION_DETAIL`        `text`         the text of the exception\'s detail
                                              message, if any

  `PG_EXCEPTION_HINT`          `text`         the text of the exception\'s hint
                                              message, if any

  `PG_EXCEPTION_CONTEXT`       `text`         line(s) of text describing the call
                                              stack at the time of the exception
                                              (see [Obtaining Execution Location
                                              Information](#plpgsql-call-stack))
  --------------------------------------------------------------------------------

  : Error Diagnostics Items

If the exception did not set a value for an item, an empty string will
be returned.

Here is an example:

    DECLARE
      text_var1 text;
      text_var2 text;
      text_var3 text;
    BEGIN
      -- some processing which might cause an exception
      ...
    EXCEPTION WHEN OTHERS THEN
      GET STACKED DIAGNOSTICS text_var1 = MESSAGE_TEXT,
                              text_var2 = PG_EXCEPTION_DETAIL,
                              text_var3 = PG_EXCEPTION_HINT;
    END;

Obtaining Execution Location Information {#plpgsql-call-stack}
----------------------------------------

The `GET DIAGNOSTICS` command, previously described in [Obtaining the
Result Status](#plpgsql-statements-diagnostics), retrieves information
about current execution state (whereas the `GET STACKED
    DIAGNOSTICS` command discussed above reports information about the
execution state as of a previous error). Its `PG_CONTEXT` status item is
useful for identifying the current execution location. `PG_CONTEXT`
returns a text string with line(s) of text describing the call stack.
The first line refers to the current function and currently executing
`GET DIAGNOSTICS` command. The second and any subsequent lines refer to
calling functions further up the call stack. For example:

    CREATE OR REPLACE FUNCTION outer_func() RETURNS integer AS $$
    BEGIN
      RETURN inner_func();
    END;
    $$ LANGUAGE plpgsql;

    CREATE OR REPLACE FUNCTION inner_func() RETURNS integer AS $$
    DECLARE
      stack text;
    BEGIN
      GET DIAGNOSTICS stack = PG_CONTEXT;
      RAISE NOTICE E'--- Call Stack ---\n%', stack;
      RETURN 1;
    END;
    $$ LANGUAGE plpgsql;

    SELECT outer_func();

    NOTICE:  --- Call Stack ---
    PL/pgSQL function inner_func() line 5 at GET DIAGNOSTICS
    PL/pgSQL function outer_func() line 3 at RETURN
    CONTEXT:  PL/pgSQL function outer_func() line 3 at RETURN
     outer_func
     ------------
               1
    (1 row)

`GET STACKED DIAGNOSTICS ... PG_EXCEPTION_CONTEXT` returns the same sort
of stack trace, but describing the location at which an error was
detected, rather than the current location.

Cursors {#plpgsql-cursors}
=======

cursor

in PL/pgSQL

Rather than executing a whole query at once, it is possible to set up a
cursor that encapsulates the query, and then read the query result a few
rows at a time. One reason for doing this is to avoid memory overrun
when the result contains a large number of rows. (However, PL/pgSQL
users do not normally need to worry about that, since `FOR` loops
automatically use a cursor internally to avoid memory problems.) A more
interesting usage is to return a reference to a cursor that a function
has created, allowing the caller to read the rows. This provides an
efficient way to return large row sets from functions.

Declaring Cursor Variables {#plpgsql-cursor-declarations}
--------------------------

All access to cursors in PL/pgSQL goes through cursor variables, which
are always of the special data type `refcursor`. One way to create a
cursor variable is just to declare it as a variable of type `refcursor`.
Another way is to use the cursor declaration syntax, which in general
is: name \[\[NO\] SCROLL\] CURSOR \[( arguments )\] FOR query; (`FOR`
can be replaced by `IS` for Oracle compatibility.) If `SCROLL` is
specified, the cursor will be capable of scrolling backward; if
`NO SCROLL` is specified, backward fetches will be rejected; if neither
specification appears, it is query-dependent whether backward fetches
will be allowed. arguments, if specified, is a comma-separated list of
pairs `name
     datatype` that define names to be replaced by parameter values in
the given query. The actual values to substitute for these names will be
specified later, when the cursor is opened.

Some examples:

    DECLARE
        curs1 refcursor;
        curs2 CURSOR FOR SELECT * FROM tenk1;
        curs3 CURSOR (key integer) FOR SELECT * FROM tenk1 WHERE unique1 = key;

All three of these variables have the data type `refcursor`, but the
first can be used with any query, while the second has a fully specified
query already bound to it, and the last has a parameterized query bound
to it. (`key` will be replaced by an integer parameter value when the
cursor is opened.) The variable `curs1` is said to be unbound since it
is not bound to any particular query.

The `SCROLL` option cannot be used when the cursor\'s query uses
`FOR UPDATE/SHARE`. Also, it is best to use `NO SCROLL` with a query
that involves volatile functions. The implementation of `SCROLL` assumes
that re-reading the query\'s output will give consistent results, which
a volatile function might not do.

Opening Cursors {#plpgsql-cursor-opening}
---------------

Before a cursor can be used to retrieve rows, it must be opened. (This
is the equivalent action to the SQL command `DECLARE CURSOR`.) PL/pgSQL
has three forms of the `OPEN` statement, two of which use unbound cursor
variables while the third uses a bound cursor variable.

> **Note**
>
> Bound cursor variables can also be used without explicitly opening the
> cursor, via the `FOR` statement described in [Looping through a
> Cursor\'s Result](#plpgsql-cursor-for-loop).

### `OPEN FOR` query

OPEN

unbound\_cursorvar

NO

SCROLL

FOR

query

;

The cursor variable is opened and given the specified query to execute.
The cursor cannot be open already, and it must have been declared as an
unbound cursor variable (that is, as a simple `refcursor` variable). The
query must be a `SELECT`, or something else that returns rows (such as
`EXPLAIN`). The query is treated in the same way as other SQL commands
in PL/pgSQL: PL/pgSQL variable names are substituted, and the query plan
is cached for possible reuse. When a PL/pgSQL variable is substituted
into the cursor query, the value that is substituted is the one it has
at the time of the `OPEN`; subsequent changes to the variable will not
affect the cursor\'s behavior. The `SCROLL` and `NO SCROLL` options have
the same meanings as for a bound cursor.

An example:

    OPEN curs1 FOR SELECT * FROM foo WHERE key = mykey;

### `OPEN FOR EXECUTE`

OPEN

unbound\_cursorvar

NO

SCROLL

FOR EXECUTE

query\_string

USING

expression

, \...

;

The cursor variable is opened and given the specified query to execute.
The cursor cannot be open already, and it must have been declared as an
unbound cursor variable (that is, as a simple `refcursor` variable). The
query is specified as a string expression, in the same way as in the
`EXECUTE` command. As usual, this gives flexibility so the query plan
can vary from one run to the next (see [Plan
Caching](#plpgsql-plan-caching)), and it also means that variable
substitution is not done on the command string. As with `EXECUTE`,
parameter values can be inserted into the dynamic command via `format()`
and `USING`. The `SCROLL` and `NO SCROLL` options have the same meanings
as for a bound cursor.

An example:

    OPEN curs1 FOR EXECUTE format('SELECT * FROM %I WHERE col1 = $1',tabname) USING keyvalue;

In this example, the table name is inserted into the query via
`format()`. The comparison value for `col1` is inserted via a `USING`
parameter, so it needs no quoting.

### Opening a Bound Cursor {#plpgsql-open-bound-cursor}

OPEN

bound\_cursorvar

(

argument\_name

:=

argument\_value

, \...

)

;

This form of `OPEN` is used to open a cursor variable whose query was
bound to it when it was declared. The cursor cannot be open already. A
list of actual argument value expressions must appear if and only if the
cursor was declared to take arguments. These values will be substituted
in the query.

The query plan for a bound cursor is always considered cacheable; there
is no equivalent of `EXECUTE` in this case. Notice that `SCROLL` and
`NO SCROLL` cannot be specified in `OPEN`, as the cursor\'s scrolling
behavior was already determined.

Argument values can be passed using either positional or named notation.
In positional notation, all arguments are specified in order. In named
notation, each argument\'s name is specified using `:=` to separate it
from the argument expression. Similar to calling functions, described in
[???](#sql-syntax-calling-funcs), it is also allowed to mix positional
and named notation.

Examples (these use the cursor declaration examples above):

    OPEN curs2;
    OPEN curs3(42);
    OPEN curs3(key := 42);

Because variable substitution is done on a bound cursor\'s query, there
are really two ways to pass values into the cursor: either with an
explicit argument to `OPEN`, or implicitly by referencing a PL/pgSQL
variable in the query. However, only variables declared before the bound
cursor was declared will be substituted into it. In either case the
value to be passed is determined at the time of the `OPEN`. For example,
another way to get the same effect as the `curs3` example above is

    DECLARE
        key integer;
        curs4 CURSOR FOR SELECT * FROM tenk1 WHERE unique1 = key;
    BEGIN
        key := 42;
        OPEN curs4;

Using Cursors {#plpgsql-cursor-using}
-------------

Once a cursor has been opened, it can be manipulated with the statements
described here.

These manipulations need not occur in the same function that opened the
cursor to begin with. You can return a `refcursor` value out of a
function and let the caller operate on the cursor. (Internally, a
`refcursor` value is simply the string name of a so-called portal
containing the active query for the cursor. This name can be passed
around, assigned to other `refcursor` variables, and so on, without
disturbing the portal.)

All portals are implicitly closed at transaction end. Therefore a
`refcursor` value is usable to reference an open cursor only until the
end of the transaction.

### `FETCH`

FETCH

direction

{ FROM \| IN }

cursor

INTO

target

;

`FETCH` retrieves the next row from the cursor into a target, which
might be a row variable, a record variable, or a comma-separated list of
simple variables, just like `SELECT INTO`. If there is no next row, the
target is set to NULL(s). As with `SELECT
     INTO`, the special variable `FOUND` can be checked to see whether a
row was obtained or not.

The direction clause can be any of the variants allowed in the SQL
[???](#sql-fetch) command except the ones that can fetch more than one
row; namely, it can be `NEXT`, `PRIOR`, `FIRST`, `LAST`, `ABSOLUTE`
count, `RELATIVE` count, `FORWARD`, or `BACKWARD`. Omitting direction is
the same as specifying `NEXT`. In the forms using a count, the count can
be any integer-valued expression (unlike the SQL `FETCH` command, which
only allows an integer constant). direction values that require moving
backward are likely to fail unless the cursor was declared or opened
with the `SCROLL` option.

cursor must be the name of a `refcursor` variable that references an
open cursor portal.

Examples:

    FETCH curs1 INTO rowvar;
    FETCH curs2 INTO foo, bar, baz;
    FETCH LAST FROM curs3 INTO x, y;
    FETCH RELATIVE -2 FROM curs4 INTO x;

### `MOVE`

MOVE

direction

{ FROM \| IN }

cursor

;

`MOVE` repositions a cursor without retrieving any data. `MOVE` works
exactly like the `FETCH` command, except it only repositions the cursor
and does not return the row moved to. As with `SELECT
     INTO`, the special variable `FOUND` can be checked to see whether
there was a next row to move to.

Examples:

    MOVE curs1;
    MOVE LAST FROM curs3;
    MOVE RELATIVE -2 FROM curs4;
    MOVE FORWARD 2 FROM curs4;

### `UPDATE/DELETE WHERE CURRENT OF`

UPDATE

table

SET \... WHERE CURRENT OF

cursor

; DELETE FROM

table

WHERE CURRENT OF

cursor

;

When a cursor is positioned on a table row, that row can be updated or
deleted using the cursor to identify the row. There are restrictions on
what the cursor\'s query can be (in particular, no grouping) and it\'s
best to use `FOR UPDATE` in the cursor. For more information see the
[???](#sql-declare) reference page.

An example:

    UPDATE foo SET dataval = myval WHERE CURRENT OF curs1;

### `CLOSE`

CLOSE

cursor

;

`CLOSE` closes the portal underlying an open cursor. This can be used to
release resources earlier than end of transaction, or to free up the
cursor variable to be opened again.

An example:

    CLOSE curs1;

### Returning Cursors

PL/pgSQL functions can return cursors to the caller. This is useful to
return multiple rows or columns, especially with very large result sets.
To do this, the function opens the cursor and returns the cursor name to
the caller (or simply opens the cursor using a portal name specified by
or otherwise known to the caller). The caller can then fetch rows from
the cursor. The cursor can be closed by the caller, or it will be closed
automatically when the transaction closes.

The portal name used for a cursor can be specified by the programmer or
automatically generated. To specify a portal name, simply assign a
string to the `refcursor` variable before opening it. The string value
of the `refcursor` variable will be used by `OPEN` as the name of the
underlying portal. However, if the `refcursor` variable is null, `OPEN`
automatically generates a name that does not conflict with any existing
portal, and assigns it to the `refcursor` variable.

> **Note**
>
> A bound cursor variable is initialized to the string value
> representing its name, so that the portal name is the same as the
> cursor variable name, unless the programmer overrides it by assignment
> before opening the cursor. But an unbound cursor variable defaults to
> the null value initially, so it will receive an
> automatically-generated unique name, unless overridden.

The following example shows one way a cursor name can be supplied by the
caller:

    CREATE TABLE test (col text);
    INSERT INTO test VALUES ('123');

    CREATE FUNCTION reffunc(refcursor) RETURNS refcursor AS '
    BEGIN
        OPEN $1 FOR SELECT col FROM test;
        RETURN $1;
    END;
    ' LANGUAGE plpgsql;

    BEGIN;
    SELECT reffunc('funccursor');
    FETCH ALL IN funccursor;
    COMMIT;

The following example uses automatic cursor name generation:

    CREATE FUNCTION reffunc2() RETURNS refcursor AS '
    DECLARE
        ref refcursor;
    BEGIN
        OPEN ref FOR SELECT col FROM test;
        RETURN ref;
    END;
    ' LANGUAGE plpgsql;

    -- need to be in a transaction to use cursors.
    BEGIN;
    SELECT reffunc2();

          reffunc2
    --------------------
     <unnamed cursor 1>
    (1 row)

    FETCH ALL IN "<unnamed cursor 1>";
    COMMIT;

The following example shows one way to return multiple cursors from a
single function:

    CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
    BEGIN
        OPEN $1 FOR SELECT * FROM table_1;
        RETURN NEXT $1;
        OPEN $2 FOR SELECT * FROM table_2;
        RETURN NEXT $2;
    END;
    $$ LANGUAGE plpgsql;

    -- need to be in a transaction to use cursors.
    BEGIN;

    SELECT * FROM myfunc('a', 'b');

    FETCH ALL FROM a;
    FETCH ALL FROM b;
    COMMIT;

Looping through a Cursor\'s Result {#plpgsql-cursor-for-loop}
----------------------------------

There is a variant of the `FOR` statement that allows iterating through
the rows returned by a cursor. The syntax is: \[\<\<label\>\>\] FOR
recordvar IN bound\_cursorvar \[( \[argument\_name :=\] argument\_value
\[, \...\] )\] LOOP statements END LOOP \[label\]; The cursor variable
must have been bound to some query when it was declared, and it *cannot*
be open already. The `FOR` statement automatically opens the cursor, and
it closes the cursor again when the loop exits. A list of actual
argument value expressions must appear if and only if the cursor was
declared to take arguments. These values will be substituted in the
query, in just the same way as during an `OPEN` (see [Opening a Bound
Cursor](#plpgsql-open-bound-cursor)).

The variable recordvar is automatically defined as type `record` and
exists only inside the loop (any existing definition of the variable
name is ignored within the loop). Each row returned by the cursor is
successively assigned to this record variable and the loop body is
executed.

Transaction Management {#plpgsql-transactions}
======================

In procedures invoked by the `CALL` command as well as in anonymous code
blocks (`DO` command), it is possible to end transactions using the
commands `COMMIT` and `ROLLBACK`. A new transaction is started
automatically after a transaction is ended using these commands, so
there is no separate `START
    TRANSACTION` command. (Note that `BEGIN` and `END` have different
meanings in PL/pgSQL.)

Here is a simple example:

    CREATE PROCEDURE transaction_test1()
    LANGUAGE plpgsql
    AS $$
    BEGIN
        FOR i IN 0..9 LOOP
            INSERT INTO test1 (a) VALUES (i);
            IF i % 2 = 0 THEN
                COMMIT;
            ELSE
                ROLLBACK;
            END IF;
        END LOOP;
    END;
    $$;

    CALL transaction_test1();

chained transactions

in PL/pgSQL

A new transaction starts out with default transaction characteristics
such as transaction isolation level. In cases where transactions are
committed in a loop, it might be desirable to start new transactions
automatically with the same characteristics as the previous one. The
commands `COMMIT AND CHAIN` and `ROLLBACK AND
    CHAIN` accomplish this.

Transaction control is only possible in `CALL` or `DO` invocations from
the top level or nested `CALL` or `DO` invocations without any other
intervening command. For example, if the call stack is `CALL proc1()`
RARR `CALL proc2()` RARR `CALL proc3()`, then the second and third
procedures can perform transaction control actions. But if the call
stack is `CALL proc1()` RARR `SELECT
    func2()` RARR `CALL proc3()`, then the last procedure cannot do
transaction control, because of the `SELECT` in between.

Special considerations apply to cursor loops. Consider this example:

    CREATE PROCEDURE transaction_test2()
    LANGUAGE plpgsql
    AS $$
    DECLARE
        r RECORD;
    BEGIN
        FOR r IN SELECT * FROM test2 ORDER BY x LOOP
            INSERT INTO test1 (a) VALUES (r.x);
            COMMIT;
        END LOOP;
    END;
    $$;

    CALL transaction_test2();

Normally, cursors are automatically closed at transaction commit.
However, a cursor created as part of a loop like this is automatically
converted to a holdable cursor by the first `COMMIT` or `ROLLBACK`. That
means that the cursor is fully evaluated at the first `COMMIT` or
`ROLLBACK` rather than row by row. The cursor is still removed
automatically after the loop, so this is mostly invisible to the user.

Transaction commands are not allowed in cursor loops driven by commands
that are not read-only (for example `UPDATE
    ... RETURNING`).

A transaction cannot be ended inside a block with exception handlers.

Errors and Messages {#plpgsql-errors-and-messages}
===================

Reporting Errors and Messages {#plpgsql-statements-raise}
-----------------------------

RAISE

in PL/pgSQL

reporting errors

in PL/pgSQL

Use the `RAISE` statement to report messages and raise errors. RAISE
\[level\] \'format\' \[, expression \[, \...\]\] \[USING option =
expression \[, \...\]\]; RAISE \[level\] condition\_name \[USING option
= expression \[, \...\]\]; RAISE \[level\] SQLSTATE \'sqlstate\' \[USING
option = expression \[, \...\]\]; RAISE \[level\] USING option =
expression \[, \...\]; RAISE ; The level option specifies the error
severity. Allowed levels are `DEBUG`, `LOG`, `INFO`, `NOTICE`,
`WARNING`, and `EXCEPTION`, with `EXCEPTION` being the default.
`EXCEPTION` raises an error (which normally aborts the current
transaction); the other levels only generate messages of different
priority levels. Whether messages of a particular priority are reported
to the client, written to the server log, or both is controlled by the
[???](#guc-log-min-messages) and [???](#guc-client-min-messages)
configuration variables. See [???](#runtime-config) for more
information.

After level if any, you can write a format (which must be a simple
string literal, not an expression). The format string specifies the
error message text to be reported. The format string can be followed by
optional argument expressions to be inserted into the message. Inside
the format string, `%` is replaced by the string representation of the
next optional argument\'s value. Write `%%` to emit a literal `%`. The
number of arguments must match the number of `%` placeholders in the
format string, or an error is raised during the compilation of the
function.

In this example, the value of `v_job_id` will replace the `%` in the
string:

    RAISE NOTICE 'Calling cs_create_job(%)', v_job_id;

You can attach additional information to the error report by writing
`USING` followed by option = expression items. Each expression can be
any string-valued expression. The allowed option key words are:

`MESSAGE`

:   Sets the error message text. This option can\'t be used in the form
    of `RAISE` that includes a format string before `USING`.

`DETAIL`

:   Supplies an error detail message.

`HINT`

:   Supplies a hint message.

`ERRCODE`

:   Specifies the error code (SQLSTATE) to report, either by condition
    name, as shown in [???](#errcodes-appendix), or directly as a
    five-character SQLSTATE code.

`COLUMN`; `CONSTRAINT`; `DATATYPE`; `TABLE`; `SCHEMA`

:   Supplies the name of a related object.

This example will abort the transaction with the given error message and
hint:

    RAISE EXCEPTION 'Nonexistent ID --> %', user_id
          USING HINT = 'Please check your user ID';

These two examples show equivalent ways of setting the SQLSTATE:

    RAISE 'Duplicate user ID: %', user_id USING ERRCODE = 'unique_violation';
    RAISE 'Duplicate user ID: %', user_id USING ERRCODE = '23505';

There is a second `RAISE` syntax in which the main argument is the
condition name or SQLSTATE to be reported, for example:

    RAISE division_by_zero;
    RAISE SQLSTATE '22012';

In this syntax, `USING` can be used to supply a custom error message,
detail, or hint. Another way to do the earlier example is

    RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;

Still another variant is to write `RAISE USING` or `RAISE
    level USING` and put everything else into the `USING` list.

The last variant of `RAISE` has no parameters at all. This form can only
be used inside a `BEGIN` block\'s `EXCEPTION` clause; it causes the
error currently being handled to be re-thrown.

> **Note**
>
> Before PostgreSQL 9.1, `RAISE` without parameters was interpreted as
> re-throwing the error from the block containing the active exception
> handler. Thus an `EXCEPTION` clause nested within that handler could
> not catch it, even if the `RAISE` was within the nested `EXCEPTION`
> clause\'s block. This was deemed surprising as well as being
> incompatible with Oracle\'s PL/SQL.

If no condition name nor SQLSTATE is specified in a `RAISE EXCEPTION`
command, the default is to use `ERRCODE_RAISE_EXCEPTION` (`P0001`). If
no message text is specified, the default is to use the condition name
or SQLSTATE as message text.

> **Note**
>
> When specifying an error code by SQLSTATE code, you are not limited to
> the predefined error codes, but can select any error code consisting
> of five digits and/or upper-case ASCII letters, other than `00000`. It
> is recommended that you avoid throwing error codes that end in three
> zeroes, because these are category codes and can only be trapped by
> trapping the whole category.

Checking Assertions {#plpgsql-statements-assert}
-------------------

ASSERT

in PL/pgSQL

assertions

in PL/pgSQL

plpgsql.check\_asserts

configuration parameter

The `ASSERT` statement is a convenient shorthand for inserting debugging
checks into PL/pgSQL functions. ASSERT condition \[, message\]; The
condition is a Boolean expression that is expected to always evaluate to
true; if it does, the `ASSERT` statement does nothing further. If the
result is false or null, then an `ASSERT_FAILURE` exception is raised.
(If an error occurs while evaluating the condition, it is reported as a
normal error.)

If the optional message is provided, it is an expression whose result
(if not null) replaces the default error message text "assertion
failed", should the condition fail. The message expression is not
evaluated in the normal case where the assertion succeeds.

Testing of assertions can be enabled or disabled via the configuration
parameter `plpgsql.check_asserts`, which takes a Boolean value; the
default is `on`. If this parameter is `off` then `ASSERT` statements do
nothing.

Note that `ASSERT` is meant for detecting program bugs, not for
reporting ordinary error conditions. Use the `RAISE` statement,
described above, for that.

Trigger Functions {#plpgsql-trigger}
=================

trigger

in PL/pgSQL

PL/pgSQL can be used to define trigger functions on data changes or
database events. A trigger function is created with the
`CREATE FUNCTION` command, declaring it as a function with no arguments
and a return type of `trigger` (for data change triggers) or
`event_trigger` (for database event triggers). Special local variables
named `TG_something` are automatically defined to describe the condition
that triggered the call.

Triggers on Data Changes {#plpgsql-dml-trigger}
------------------------

A [data change trigger](#triggers) is declared as a function with no
arguments and a return type of `trigger`. Note that the function must be
declared with no arguments even if it expects to receive some arguments
specified in `CREATE TRIGGER` MDASH such arguments are passed via
`TG_ARGV`, as described below.

When a PL/pgSQL function is called as a trigger, several special
variables are created automatically in the top-level block. They are:

`NEW`

:   Data type `RECORD`; variable holding the new database row for
    `INSERT`/`UPDATE` operations in row-level triggers. This variable is
    null in statement-level triggers and for `DELETE` operations.

`OLD`

:   Data type `RECORD`; variable holding the old database row for
    `UPDATE`/`DELETE` operations in row-level triggers. This variable is
    null in statement-level triggers and for `INSERT` operations.

`TG_NAME`

:   Data type `name`; variable that contains the name of the trigger
    actually fired.

`TG_WHEN`

:   Data type `text`; a string of `BEFORE`, `AFTER`, or `INSTEAD OF`,
    depending on the trigger\'s definition.

`TG_LEVEL`

:   Data type `text`; a string of either `ROW` or `STATEMENT` depending
    on the trigger\'s definition.

`TG_OP`

:   Data type `text`; a string of `INSERT`, `UPDATE`, `DELETE`, or
    `TRUNCATE` telling for which operation the trigger was fired.

`TG_RELID`

:   Data type `oid`; the object ID of the table that caused the trigger
    invocation.

`TG_RELNAME`

:   Data type `name`; the name of the table that caused the trigger
    invocation. This is now deprecated, and could disappear in a future
    release. Use `TG_TABLE_NAME` instead.

`TG_TABLE_NAME`

:   Data type `name`; the name of the table that caused the trigger
    invocation.

`TG_TABLE_SCHEMA`

:   Data type `name`; the name of the schema of the table that caused
    the trigger invocation.

`TG_NARGS`

:   Data type `integer`; the number of arguments given to the trigger
    function in the `CREATE TRIGGER` statement.

`TG_ARGV[]`

:   Data type array of `text`; the arguments from the `CREATE TRIGGER`
    statement. The index counts from 0. Invalid indexes (less than 0 or
    greater than or equal to `tg_nargs`) result in a null value.

A trigger function must return either `NULL` or a record/row value
having exactly the structure of the table the trigger was fired for.

Row-level triggers fired `BEFORE` can return null to signal the trigger
manager to skip the rest of the operation for this row (i.e., subsequent
triggers are not fired, and the `INSERT`/`UPDATE`/`DELETE` does not
occur for this row). If a nonnull value is returned then the operation
proceeds with that row value. Returning a row value different from the
original value of `NEW` alters the row that will be inserted or updated.
Thus, if the trigger function wants the triggering action to succeed
normally without altering the row value, `NEW` (or a value equal
thereto) has to be returned. To alter the row to be stored, it is
possible to replace single values directly in `NEW` and return the
modified `NEW`, or to build a complete new record/row to return. In the
case of a before-trigger on `DELETE`, the returned value has no direct
effect, but it has to be nonnull to allow the trigger action to proceed.
Note that `NEW` is null in `DELETE` triggers, so returning that is
usually not sensible. The usual idiom in `DELETE` triggers is to return
`OLD`.

`INSTEAD OF` triggers (which are always row-level triggers, and may only
be used on views) can return null to signal that they did not perform
any updates, and that the rest of the operation for this row should be
skipped (i.e., subsequent triggers are not fired, and the row is not
counted in the rows-affected status for the surrounding
`INSERT`/`UPDATE`/`DELETE`). Otherwise a nonnull value should be
returned, to signal that the trigger performed the requested operation.
For `INSERT` and `UPDATE` operations, the return value should be `NEW`,
which the trigger function may modify to support `INSERT RETURNING` and
`UPDATE RETURNING` (this will also affect the row value passed to any
subsequent triggers, or passed to a special `EXCLUDED` alias reference
within an `INSERT` statement with an `ON CONFLICT DO
    UPDATE` clause). For `DELETE` operations, the return value should be
`OLD`.

The return value of a row-level trigger fired `AFTER` or a
statement-level trigger fired `BEFORE` or `AFTER` is always ignored; it
might as well be null. However, any of these types of triggers might
still abort the entire operation by raising an error.

[example\_title](#plpgsql-trigger-example) shows an example of a trigger
function in PL/pgSQL.

This example trigger ensures that any time a row is inserted or updated
in the table, the current user name and time are stamped into the row.
And it checks that an employee\'s name is given and that the salary is a
positive value.

    CREATE TABLE emp (
        empname text,
        salary integer,
        last_date timestamp,
        last_user text
    );

    CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
        BEGIN
            -- Check that empname and salary are given
            IF NEW.empname IS NULL THEN
                RAISE EXCEPTION 'empname cannot be null';
            END IF;
            IF NEW.salary IS NULL THEN
                RAISE EXCEPTION '% cannot have null salary', NEW.empname;
            END IF;

            -- Who works for us when they must pay for it?
            IF NEW.salary < 0 THEN
                RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
            END IF;

            -- Remember who changed the payroll when
            NEW.last_date := current_timestamp;
            NEW.last_user := current_user;
            RETURN NEW;
        END;
    $emp_stamp$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
        FOR EACH ROW EXECUTE FUNCTION emp_stamp();

Another way to log changes to a table involves creating a new table that
holds a row for each insert, update, or delete that occurs. This
approach can be thought of as auditing changes to a table.
[example\_title](#plpgsql-trigger-audit-example) shows an example of an
audit trigger function in PL/pgSQL.

This example trigger ensures that any insert, update or delete of a row
in the `emp` table is recorded (i.e., audited) in the `emp_audit` table.
The current time and user name are stamped into the row, together with
the type of operation performed on it.

    CREATE TABLE emp (
        empname           text NOT NULL,
        salary            integer
    );

    CREATE TABLE emp_audit(
        operation         char(1)   NOT NULL,
        stamp             timestamp NOT NULL,
        userid            text      NOT NULL,
        empname           text      NOT NULL,
        salary integer
    );

    CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
        BEGIN
            --
            -- Create a row in emp_audit to reflect the operation performed on emp,
            -- making use of the special variable TG_OP to work out the operation.
            --
            IF (TG_OP = 'DELETE') THEN
                INSERT INTO emp_audit SELECT 'D', now(), user, OLD.*;
            ELSIF (TG_OP = 'UPDATE') THEN
                INSERT INTO emp_audit SELECT 'U', now(), user, NEW.*;
            ELSIF (TG_OP = 'INSERT') THEN
                INSERT INTO emp_audit SELECT 'I', now(), user, NEW.*;
            END IF;
            RETURN NULL; -- result is ignored since this is an AFTER trigger
        END;
    $emp_audit$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_audit
    AFTER INSERT OR UPDATE OR DELETE ON emp
        FOR EACH ROW EXECUTE FUNCTION process_emp_audit();

A variation of the previous example uses a view joining the main table
to the audit table, to show when each entry was last modified. This
approach still records the full audit trail of changes to the table, but
also presents a simplified view of the audit trail, showing just the
last modified timestamp derived from the audit trail for each entry.
[example\_title](#plpgsql-view-trigger-audit-example) shows an example
of an audit trigger on a view in PL/pgSQL.

This example uses a trigger on the view to make it updatable, and ensure
that any insert, update or delete of a row in the view is recorded
(i.e., audited) in the `emp_audit` table. The current time and user name
are recorded, together with the type of operation performed, and the
view displays the last modified time of each row.

    CREATE TABLE emp (
        empname           text PRIMARY KEY,
        salary            integer
    );

    CREATE TABLE emp_audit(
        operation         char(1)   NOT NULL,
        userid            text      NOT NULL,
        empname           text      NOT NULL,
        salary            integer,
        stamp             timestamp NOT NULL
    );

    CREATE VIEW emp_view AS
        SELECT e.empname,
               e.salary,
               max(ea.stamp) AS last_updated
          FROM emp e
          LEFT JOIN emp_audit ea ON ea.empname = e.empname
         GROUP BY 1, 2;

    CREATE OR REPLACE FUNCTION update_emp_view() RETURNS TRIGGER AS $$
        BEGIN
            --
            -- Perform the required operation on emp, and create a row in emp_audit
            -- to reflect the change made to emp.
            --
            IF (TG_OP = 'DELETE') THEN
                DELETE FROM emp WHERE empname = OLD.empname;
                IF NOT FOUND THEN RETURN NULL; END IF;

                OLD.last_updated = now();
                INSERT INTO emp_audit VALUES('D', user, OLD.*);
                RETURN OLD;
            ELSIF (TG_OP = 'UPDATE') THEN
                UPDATE emp SET salary = NEW.salary WHERE empname = OLD.empname;
                IF NOT FOUND THEN RETURN NULL; END IF;

                NEW.last_updated = now();
                INSERT INTO emp_audit VALUES('U', user, NEW.*);
                RETURN NEW;
            ELSIF (TG_OP = 'INSERT') THEN
                INSERT INTO emp VALUES(NEW.empname, NEW.salary);

                NEW.last_updated = now();
                INSERT INTO emp_audit VALUES('I', user, NEW.*);
                RETURN NEW;
            END IF;
        END;
    $$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_audit
    INSTEAD OF INSERT OR UPDATE OR DELETE ON emp_view
        FOR EACH ROW EXECUTE FUNCTION update_emp_view();

One use of triggers is to maintain a summary table of another table. The
resulting summary can be used in place of the original table for certain
queries MDASH often with vastly reduced run times. This technique is
commonly used in Data Warehousing, where the tables of measured or
observed data (called fact tables) might be extremely large.
[example\_title](#plpgsql-trigger-summary-example) shows an example of a
trigger function in PL/pgSQL that maintains a summary table for a fact
table in a data warehouse.

The schema detailed here is partly based on the *Grocery Store* example
from *The Data Warehouse Toolkit* by Ralph Kimball.

    --
    -- Main tables - time dimension and sales fact.
    --
    CREATE TABLE time_dimension (
        time_key                    integer NOT NULL,
        day_of_week                 integer NOT NULL,
        day_of_month                integer NOT NULL,
        month                       integer NOT NULL,
        quarter                     integer NOT NULL,
        year                        integer NOT NULL
    );
    CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key);

    CREATE TABLE sales_fact (
        time_key                    integer NOT NULL,
        product_key                 integer NOT NULL,
        store_key                   integer NOT NULL,
        amount_sold                 numeric(12,2) NOT NULL,
        units_sold                  integer NOT NULL,
        amount_cost                 numeric(12,2) NOT NULL
    );
    CREATE INDEX sales_fact_time ON sales_fact(time_key);

    --
    -- Summary table - sales by time.
    --
    CREATE TABLE sales_summary_bytime (
        time_key                    integer NOT NULL,
        amount_sold                 numeric(15,2) NOT NULL,
        units_sold                  numeric(12) NOT NULL,
        amount_cost                 numeric(15,2) NOT NULL
    );
    CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key);

    --
    -- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE.
    --
    CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER
    AS $maint_sales_summary_bytime$
        DECLARE
            delta_time_key          integer;
            delta_amount_sold       numeric(15,2);
            delta_units_sold        numeric(12);
            delta_amount_cost       numeric(15,2);
        BEGIN

            -- Work out the increment/decrement amount(s).
            IF (TG_OP = 'DELETE') THEN

                delta_time_key = OLD.time_key;
                delta_amount_sold = -1 * OLD.amount_sold;
                delta_units_sold = -1 * OLD.units_sold;
                delta_amount_cost = -1 * OLD.amount_cost;

            ELSIF (TG_OP = 'UPDATE') THEN

                -- forbid updates that change the time_key -
                -- (probably not too onerous, as DELETE + INSERT is how most
                -- changes will be made).
                IF ( OLD.time_key != NEW.time_key) THEN
                    RAISE EXCEPTION 'Update of time_key : % -> % not allowed',
                                                          OLD.time_key, NEW.time_key;
                END IF;

                delta_time_key = OLD.time_key;
                delta_amount_sold = NEW.amount_sold - OLD.amount_sold;
                delta_units_sold = NEW.units_sold - OLD.units_sold;
                delta_amount_cost = NEW.amount_cost - OLD.amount_cost;

            ELSIF (TG_OP = 'INSERT') THEN

                delta_time_key = NEW.time_key;
                delta_amount_sold = NEW.amount_sold;
                delta_units_sold = NEW.units_sold;
                delta_amount_cost = NEW.amount_cost;

            END IF;


            -- Insert or update the summary row with the new values.
            <<insert_update>>
            LOOP
                UPDATE sales_summary_bytime
                    SET amount_sold = amount_sold + delta_amount_sold,
                        units_sold = units_sold + delta_units_sold,
                        amount_cost = amount_cost + delta_amount_cost
                    WHERE time_key = delta_time_key;

                EXIT insert_update WHEN found;

                BEGIN
                    INSERT INTO sales_summary_bytime (
                                time_key,
                                amount_sold,
                                units_sold,
                                amount_cost)
                        VALUES (
                                delta_time_key,
                                delta_amount_sold,
                                delta_units_sold,
                                delta_amount_cost
                               );

                    EXIT insert_update;

                EXCEPTION
                    WHEN UNIQUE_VIOLATION THEN
                        -- do nothing
                END;
            END LOOP insert_update;

            RETURN NULL;

        END;
    $maint_sales_summary_bytime$ LANGUAGE plpgsql;

    CREATE TRIGGER maint_sales_summary_bytime
    AFTER INSERT OR UPDATE OR DELETE ON sales_fact
        FOR EACH ROW EXECUTE FUNCTION maint_sales_summary_bytime();

    INSERT INTO sales_fact VALUES(1,1,1,10,3,15);
    INSERT INTO sales_fact VALUES(1,2,1,20,5,35);
    INSERT INTO sales_fact VALUES(2,2,1,40,15,135);
    INSERT INTO sales_fact VALUES(2,3,1,10,1,13);
    SELECT * FROM sales_summary_bytime;
    DELETE FROM sales_fact WHERE product_key = 1;
    SELECT * FROM sales_summary_bytime;
    UPDATE sales_fact SET units_sold = units_sold * 2;
    SELECT * FROM sales_summary_bytime;

`AFTER` triggers can also make use of transition tables to inspect the
entire set of rows changed by the triggering statement. The
`CREATE TRIGGER` command assigns names to one or both transition tables,
and then the function can refer to those names as though they were
read-only temporary tables.
[example\_title](#plpgsql-trigger-audit-transition-example) shows an
example.

This example produces the same results as
[example\_title](#plpgsql-trigger-audit-example), but instead of using a
trigger that fires for every row, it uses a trigger that fires once per
statement, after collecting the relevant information in a transition
table. This can be significantly faster than the row-trigger approach
when the invoking statement has modified many rows. Notice that we must
make a separate trigger declaration for each kind of event, since the
`REFERENCING` clauses must be different for each case. But this does not
stop us from using a single trigger function if we choose. (In practice,
it might be better to use three separate functions and avoid the
run-time tests on `TG_OP`.)

    CREATE TABLE emp (
        empname           text NOT NULL,
        salary            integer
    );

    CREATE TABLE emp_audit(
        operation         char(1)   NOT NULL,
        stamp             timestamp NOT NULL,
        userid            text      NOT NULL,
        empname           text      NOT NULL,
        salary integer
    );

    CREATE OR REPLACE FUNCTION process_emp_audit() RETURNS TRIGGER AS $emp_audit$
        BEGIN
            --
            -- Create rows in emp_audit to reflect the operations performed on emp,
            -- making use of the special variable TG_OP to work out the operation.
            --
            IF (TG_OP = 'DELETE') THEN
                INSERT INTO emp_audit
                    SELECT 'D', now(), user, o.* FROM old_table o;
            ELSIF (TG_OP = 'UPDATE') THEN
                INSERT INTO emp_audit
                    SELECT 'U', now(), user, n.* FROM new_table n;
            ELSIF (TG_OP = 'INSERT') THEN
                INSERT INTO emp_audit
                    SELECT 'I', now(), user, n.* FROM new_table n;
            END IF;
            RETURN NULL; -- result is ignored since this is an AFTER trigger
        END;
    $emp_audit$ LANGUAGE plpgsql;

    CREATE TRIGGER emp_audit_ins
        AFTER INSERT ON emp
        REFERENCING NEW TABLE AS new_table
        FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
    CREATE TRIGGER emp_audit_upd
        AFTER UPDATE ON emp
        REFERENCING OLD TABLE AS old_table NEW TABLE AS new_table
        FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();
    CREATE TRIGGER emp_audit_del
        AFTER DELETE ON emp
        REFERENCING OLD TABLE AS old_table
        FOR EACH STATEMENT EXECUTE FUNCTION process_emp_audit();

Triggers on Events {#plpgsql-event-trigger}
------------------

PL/pgSQL can be used to define [event triggers](#event-triggers).
PostgreSQL requires that a function that is to be called as an event
trigger must be declared as a function with no arguments and a return
type of `event_trigger`.

When a PL/pgSQL function is called as an event trigger, several special
variables are created automatically in the top-level block. They are:

`TG_EVENT`

:   Data type `text`; a string representing the event the trigger is
    fired for.

`TG_TAG`

:   Data type `text`; variable that contains the command tag for which
    the trigger is fired.

[example\_title](#plpgsql-event-trigger-example) shows an example of an
event trigger function in PL/pgSQL.

This example trigger simply raises a `NOTICE` message each time a
supported command is executed.

    CREATE OR REPLACE FUNCTION snitch() RETURNS event_trigger AS $$
    BEGIN
        RAISE NOTICE 'snitch: % %', tg_event, tg_tag;
    END;
    $$ LANGUAGE plpgsql;

    CREATE EVENT TRIGGER snitch ON ddl_command_start EXECUTE FUNCTION snitch();

PL/pgSQL under the Hood {#plpgsql-implementation}
=======================

This section discusses some implementation details that are frequently
important for PL/pgSQL users to know.

Variable Substitution {#plpgsql-var-subst}
---------------------

SQL statements and expressions within a PL/pgSQL function can refer to
variables and parameters of the function. Behind the scenes, PL/pgSQL
substitutes query parameters for such references. Query parameters will
only be substituted in places where they are syntactically permissible.
As an extreme case, consider this example of poor programming style:

    INSERT INTO foo (foo) VALUES (foo(foo));

The first occurrence of `foo` must syntactically be a table name, so it
will not be substituted, even if the function has a variable named
`foo`. The second occurrence must be the name of a column of that table,
so it will not be substituted either. Likewise the third occurrence must
be a function name, so it also will not be substituted for. Only the
last occurrence is a candidate to be a reference to a variable of the
PL/pgSQL function.

Another way to understand this is that variable substitution can only
insert data values into an SQL command; it cannot dynamically change
which database objects are referenced by the command. (If you want to do
that, you must build a command string dynamically, as explained in
[Executing Dynamic Commands](#plpgsql-statements-executing-dyn).)

Since the names of variables are syntactically no different from the
names of table columns, there can be ambiguity in statements that also
refer to tables: is a given name meant to refer to a table column, or a
variable? Let\'s change the previous example to

    INSERT INTO dest (col) SELECT foo + bar FROM src;

Here, `dest` and `src` must be table names, and `col` must be a column
of `dest`, but `foo` and `bar` might reasonably be either variables of
the function or columns of `src`.

By default, PL/pgSQL will report an error if a name in an SQL statement
could refer to either a variable or a table column. You can fix such a
problem by renaming the variable or column, or by qualifying the
ambiguous reference, or by telling PL/pgSQL which interpretation to
prefer.

The simplest solution is to rename the variable or column. A common
coding rule is to use a different naming convention for PL/pgSQL
variables than you use for column names. For example, if you
consistently name function variables `v_something` while none of your
column names start with `v_`, no conflicts will occur.

Alternatively you can qualify ambiguous references to make them clear.
In the above example, `src.foo` would be an unambiguous reference to the
table column. To create an unambiguous reference to a variable, declare
it in a labeled block and use the block\'s label (see [Structure of
](#plpgsql-structure)). For example,

    <<block>>
    DECLARE
        foo int;
    BEGIN
        foo := ...;
        INSERT INTO dest (col) SELECT block.foo + bar FROM src;

Here `block.foo` means the variable even if there is a column `foo` in
`src`. Function parameters, as well as special variables such as
`FOUND`, can be qualified by the function\'s name, because they are
implicitly declared in an outer block labeled with the function\'s name.

Sometimes it is impractical to fix all the ambiguous references in a
large body of PL/pgSQL code. In such cases you can specify that PL/pgSQL
should resolve ambiguous references as the variable (which is compatible
with PL/pgSQL\'s behavior before PostgreSQL 9.0), or as the table column
(which is compatible with some other systems such as Oracle).

plpgsql.variable\_conflict

configuration parameter

To change this behavior on a system-wide basis, set the configuration
parameter `plpgsql.variable_conflict` to one of `error`, `use_variable`,
or `use_column` (where `error` is the factory default). This parameter
affects subsequent compilations of statements in PL/pgSQL functions, but
not statements already compiled in the current session. Because changing
this setting can cause unexpected changes in the behavior of PL/pgSQL
functions, it can only be changed by a superuser.

You can also set the behavior on a function-by-function basis, by
inserting one of these special commands at the start of the function
text:

    #variable_conflict error
    #variable_conflict use_variable
    #variable_conflict use_column

These commands affect only the function they are written in, and
override the setting of `plpgsql.variable_conflict`. An example is

    CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
        #variable_conflict use_variable
        DECLARE
            curtime timestamp := now();
        BEGIN
            UPDATE users SET last_modified = curtime, comment = comment
              WHERE users.id = id;
        END;
    $$ LANGUAGE plpgsql;

In the `UPDATE` command, `curtime`, `comment`, and `id` will refer to
the function\'s variable and parameters whether or not `users` has
columns of those names. Notice that we had to qualify the reference to
`users.id` in the `WHERE` clause to make it refer to the table column.
But we did not have to qualify the reference to `comment` as a target in
the `UPDATE` list, because syntactically that must be a column of
`users`. We could write the same function without depending on the
`variable_conflict` setting in this way:

    CREATE FUNCTION stamp_user(id int, comment text) RETURNS void AS $$
        <<fn>>
        DECLARE
            curtime timestamp := now();
        BEGIN
            UPDATE users SET last_modified = fn.curtime, comment = stamp_user.comment
              WHERE users.id = stamp_user.id;
        END;
    $$ LANGUAGE plpgsql;

Variable substitution does not happen in a command string given to
`EXECUTE` or one of its variants. If you need to insert a varying value
into such a command, do so as part of constructing the string value, or
use `USING`, as illustrated in [Executing Dynamic
Commands](#plpgsql-statements-executing-dyn).

Variable substitution currently works only in `SELECT`, `INSERT`,
`UPDATE`, `DELETE`, and commands containing one of these (such as
`EXPLAIN` and `CREATE TABLE
    ... AS SELECT`), because the main SQL engine allows query parameters
only in these commands. To use a non-constant name or value in other
statement types (generically called utility statements), you must
construct the utility statement as a string and `EXECUTE` it.

Plan Caching {#plpgsql-plan-caching}
------------

The PL/pgSQL interpreter parses the function\'s source text and produces
an internal binary instruction tree the first time the function is
called (within each session). The instruction tree fully translates the
PL/pgSQL statement structure, but individual SQL expressions and SQL
commands used in the function are not translated immediately.

preparing a query in PL/pgSQL As each expression and SQL command is
first executed in the function, the PL/pgSQL interpreter parses and
analyzes the command to create a prepared statement, using the SPI
manager\'s `SPI_prepare` function. Subsequent visits to that expression
or command reuse the prepared statement. Thus, a function with
conditional code paths that are seldom visited will never incur the
overhead of analyzing those commands that are never executed within the
current session. A disadvantage is that errors in a specific expression
or command cannot be detected until that part of the function is reached
in execution. (Trivial syntax errors will be detected during the initial
parsing pass, but anything deeper will not be detected until execution.)

PL/pgSQL (or more precisely, the SPI manager) can furthermore attempt to
cache the execution plan associated with any particular prepared
statement. If a cached plan is not used, then a fresh execution plan is
generated on each visit to the statement, and the current parameter
values (that is, PL/pgSQL variable values) can be used to optimize the
selected plan. If the statement has no parameters, or is executed many
times, the SPI manager will consider creating a generic plan that is not
dependent on specific parameter values, and caching that for re-use.
Typically this will happen only if the execution plan is not very
sensitive to the values of the PL/pgSQL variables referenced in it. If
it is, generating a plan each time is a net win. See [???](#sql-prepare)
for more information about the behavior of prepared statements.

Because PL/pgSQL saves prepared statements and sometimes execution plans
in this way, SQL commands that appear directly in a PL/pgSQL function
must refer to the same tables and columns on every execution; that is,
you cannot use a parameter as the name of a table or column in an SQL
command. To get around this restriction, you can construct dynamic
commands using the PL/pgSQL `EXECUTE` statement MDASH at the price of
performing new parse analysis and constructing a new execution plan on
every execution.

The mutable nature of record variables presents another problem in this
connection. When fields of a record variable are used in expressions or
statements, the data types of the fields must not change from one call
of the function to the next, since each expression will be analyzed
using the data type that is present when the expression is first
reached. `EXECUTE` can be used to get around this problem when
necessary.

If the same function is used as a trigger for more than one table,
PL/pgSQL prepares and caches statements independently for each such
table MDASH that is, there is a cache for each trigger function and
table combination, not just for each function. This alleviates some of
the problems with varying data types; for instance, a trigger function
will be able to work successfully with a column named `key` even if it
happens to have different types in different tables.

Likewise, functions having polymorphic argument types have a separate
statement cache for each combination of actual argument types they have
been invoked for, so that data type differences do not cause unexpected
failures.

Statement caching can sometimes have surprising effects on the
interpretation of time-sensitive values. For example there is a
difference between what these two functions do:

    CREATE FUNCTION logfunc1(logtxt text) RETURNS void AS $$
        BEGIN
            INSERT INTO logtable VALUES (logtxt, 'now');
        END;
    $$ LANGUAGE plpgsql;

and:

    CREATE FUNCTION logfunc2(logtxt text) RETURNS void AS $$
        DECLARE
            curtime timestamp;
        BEGIN
            curtime := 'now';
            INSERT INTO logtable VALUES (logtxt, curtime);
        END;
    $$ LANGUAGE plpgsql;

In the case of `logfunc1`, the PostgreSQL main parser knows when
analyzing the `INSERT` that the string `'now'` should be interpreted as
`timestamp`, because the target column of `logtable` is of that type.
Thus, `'now'` will be converted to a `timestamp` constant when the
`INSERT` is analyzed, and then used in all invocations of `logfunc1`
during the lifetime of the session. Needless to say, this isn\'t what
the programmer wanted. A better idea is to use the `now()` or
`current_timestamp` function.

In the case of `logfunc2`, the PostgreSQL main parser does not know what
type `'now'` should become and therefore it returns a data value of type
`text` containing the string `now`. During the ensuing assignment to the
local variable `curtime`, the PL/pgSQL interpreter casts this string to
the `timestamp` type by calling the `textout` and `timestamp_in`
functions for the conversion. So, the computed time stamp is updated on
each execution as the programmer expects. Even though this happens to
work as expected, it\'s not terribly efficient, so use of the `now()`
function would still be a better idea.

Tips for Developing in PL/pgSQL {#plpgsql-development-tips}
===============================

One good way to develop in PL/pgSQL is to use the text editor of your
choice to create your functions, and in another window, use psql to load
and test those functions. If you are doing it this way, it is a good
idea to write the function using `CREATE OR
    REPLACE FUNCTION`. That way you can just reload the file to update
the function definition. For example:

    CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $$
              ....
    $$ LANGUAGE plpgsql;

While running psql, you can load or reload such a function definition
file with:

    \i filename.sql

and then immediately issue SQL commands to test the function.

Another good way to develop in PL/pgSQL is with a GUI database access
tool that facilitates development in a procedural language. One example
of such a tool is pgAdmin, although others exist. These tools often
provide convenient features such as escaping single quotes and making it
easier to recreate and debug functions.

Handling of Quotation Marks {#plpgsql-quote-tips}
---------------------------

The code of a PL/pgSQL function is specified in `CREATE FUNCTION` as a
string literal. If you write the string literal in the ordinary way with
surrounding single quotes, then any single quotes inside the function
body must be doubled; likewise any backslashes must be doubled (assuming
escape string syntax is used). Doubling quotes is at best tedious, and
in more complicated cases the code can become downright
incomprehensible, because you can easily find yourself needing half a
dozen or more adjacent quote marks. It\'s recommended that you instead
write the function body as a "dollar-quoted" string literal (see
[???](#sql-syntax-dollar-quoting)). In the dollar-quoting approach, you
never double any quote marks, but instead take care to choose a
different dollar-quoting delimiter for each level of nesting you need.
For example, you might write the `CREATE
    FUNCTION` command as:

    CREATE OR REPLACE FUNCTION testfunc(integer) RETURNS integer AS $PROC$
              ....
    $PROC$ LANGUAGE plpgsql;

Within this, you might use quote marks for simple literal strings in SQL
commands and `$$` to delimit fragments of SQL commands that you are
assembling as strings. If you need to quote text that includes `$$`, you
could use `$Q$`, and so on.

The following chart shows what you have to do when writing quote marks
without dollar quoting. It might be useful when translating pre-dollar
quoting code into something more comprehensible.

1 quotation mark

:   To begin and end the function body, for example:

        CREATE FUNCTION foo() RETURNS integer AS '
                  ....
        ' LANGUAGE plpgsql;

    Anywhere within a single-quoted function body, quote marks *must*
    appear in pairs.

2 quotation marks

:   For string literals inside the function body, for example:

        a_output := ''Blah'';
        SELECT * FROM users WHERE f_name=''foobar'';

    In the dollar-quoting approach, you\'d just write:

        a_output := 'Blah';
        SELECT * FROM users WHERE f_name='foobar';

    which is exactly what the PL/pgSQL parser would see in either case.

4 quotation marks

:   When you need a single quotation mark in a string constant inside
    the function body, for example:

        a_output := a_output || '' AND name LIKE ''''foobar'''' AND xyz''

    The value actually appended to `a_output` would be:
    ` AND name LIKE 'foobar' AND xyz`.

    In the dollar-quoting approach, you\'d write:

        a_output := a_output || $$ AND name LIKE 'foobar' AND xyz$$

    being careful that any dollar-quote delimiters around this are not
    just `$$`.

6 quotation marks

:   When a single quotation mark in a string inside the function body is
    adjacent to the end of that string constant, for example:

        a_output := a_output || '' AND name LIKE ''''foobar''''''

    The value appended to `a_output` would then be:
    ` AND name LIKE 'foobar'`.

    In the dollar-quoting approach, this becomes:

        a_output := a_output || $$ AND name LIKE 'foobar'$$

10 quotation marks

:   When you want two single quotation marks in a string constant (which
    accounts for 8 quotation marks) and this is adjacent to the end of
    that string constant (2 more). You will probably only need that if
    you are writing a function that generates other functions, as in
    [example\_title](#plpgsql-porting-ex2). For example:

        a_output := a_output || '' if v_'' ||
            referrer_keys.kind || '' like ''''''''''
            || referrer_keys.key_string || ''''''''''
            then return ''''''  || referrer_keys.referrer_type
            || ''''''; end if;'';

    The value of `a_output` would then be:

        if v_... like ''...'' then return ''...''; end if;

    In the dollar-quoting approach, this becomes:

        a_output := a_output || $$ if v_$$ || referrer_keys.kind || $$ like '$$
            || referrer_keys.key_string || $$'
            then return '$$  || referrer_keys.referrer_type
            || $$'; end if;$$;

    where we assume we only need to put single quote marks into
    `a_output`, because it will be re-quoted before use.

Additional Compile-Time and Run-Time Checks {#plpgsql-extra-checks}
-------------------------------------------

To aid the user in finding instances of simple but common problems
before they cause harm, PL/pgSQL provides additional checks. When
enabled, depending on the configuration, they can be used to emit either
a `WARNING` or an `ERROR` during the compilation of a function. A
function which has received a `WARNING` can be executed without
producing further messages, so you are advised to test in a separate
development environment.

Setting `plpgsql.extra_warnings`, or `plpgsql.extra_errors`, as
appropriate, to `"all"` is encouraged in development and/or testing
environments.

These additional checks are enabled through the configuration variables
`plpgsql.extra_warnings` for warnings and `plpgsql.extra_errors` for
errors. Both can be set either to a comma-separated list of checks,
`"none"` or `"all"`. The default is `"none"`. Currently the list of
available checks includes:

`shadowed_variables`

:   Checks if a declaration shadows a previously defined variable.

`strict_multi_assignment`

:   Some PL/PgSQL commands allow assigning values to more than one
    variable at a time, such as `SELECT INTO`. Typically, the number of
    target variables and the number of source variables should match,
    though PL/PgSQL will use `NULL` for missing values and extra
    variables are ignored. Enabling this check will cause PL/PgSQL to
    throw a `WARNING` or `ERROR` whenever the number of target variables
    and the number of source variables are different.

`too_many_rows`

:   Enabling this check will cause PL/PgSQL to check if a given query
    returns more than one row when an `INTO` clause is used. As an
    `INTO` statement will only ever use one row, having a query return
    multiple rows is generally either inefficient and/or
    nondeterministic and therefore is likely an error.

The following example shows the effect of `plpgsql.extra_warnings` set
to `shadowed_variables`:

    SET plpgsql.extra_warnings TO 'shadowed_variables';

    CREATE FUNCTION foo(f1 int) RETURNS int AS $$
    DECLARE
    f1 int;
    BEGIN
    RETURN f1;
    END;
    $$ LANGUAGE plpgsql;
    WARNING:  variable "f1" shadows a previously defined variable
    LINE 3: f1 int;
            ^
    CREATE FUNCTION

The below example shows the effects of setting `plpgsql.extra_warnings`
to `strict_multi_assignment`:

    SET plpgsql.extra_warnings TO 'strict_multi_assignment';

    CREATE OR REPLACE FUNCTION public.foo()
     RETURNS void
     LANGUAGE plpgsql
    AS $$
    DECLARE
      x int;
      y int;
    BEGIN
      SELECT 1 INTO x, y;
      SELECT 1, 2 INTO x, y;
      SELECT 1, 2, 3 INTO x, y;
    END;
    $$;

    SELECT foo();
    WARNING:  number of source and target fields in assignment does not match
    DETAIL:  strict_multi_assignment check of extra_warnings is active.
    HINT:  Make sure the query returns the exact list of columns.
    WARNING:  number of source and target fields in assignment does not match
    DETAIL:  strict_multi_assignment check of extra_warnings is active.
    HINT:  Make sure the query returns the exact list of columns.

     foo 
    -----
     
    (1 row)

Porting from Oracle PL/SQL {#plpgsql-porting}
==========================

Oracle

porting from PL/SQL to PL/pgSQL

PL/SQL (Oracle)

porting to PL/pgSQL

This section explains differences between PostgreSQL\'s PL/pgSQL
language and Oracle\'s PL/SQL language, to help developers who port
applications from Oracle to PostgreSQL.

PL/pgSQL is similar to PL/SQL in many aspects. It is a block-structured,
imperative language, and all variables have to be declared. Assignments,
loops, and conditionals are similar. The main differences you should
keep in mind when porting from PL/SQL to PL/pgSQL are:

-   If a name used in an SQL command could be either a column name of a
    table used in the command or a reference to a variable of the
    function, PL/SQL treats it as a column name. By default, PL/pgSQL
    will throw an error complaining that the name is ambiguous. You can
    specify `plpgsql.variable_conflict` = `use_column` to change this
    behavior to match PL/SQL, as explained in [Variable
    Substitution](#plpgsql-var-subst). It\'s often best to avoid such
    ambiguities in the first place, but if you have to port a large
    amount of code that depends on this behavior, setting
    `variable_conflict` may be the best solution.

-   In PostgreSQL the function body must be written as a string literal.
    Therefore you need to use dollar quoting or escape single quotes in
    the function body. (See [Handling of Quotation
    Marks](#plpgsql-quote-tips).)

-   Data type names often need translation. For example, in Oracle
    string values are commonly declared as being of type `varchar2`,
    which is a non-SQL-standard type. In PostgreSQL, use type `varchar`
    or `text` instead. Similarly, replace type `number` with `numeric`,
    or use some other numeric data type if there\'s a more appropriate
    one.

-   Instead of packages, use schemas to organize your functions into
    groups.

-   Since there are no packages, there are no package-level variables
    either. This is somewhat annoying. You can keep per-session state in
    temporary tables instead.

-   Integer `FOR` loops with `REVERSE` work differently: PL/SQL counts
    down from the second number to the first, while PL/pgSQL counts down
    from the first number to the second, requiring the loop bounds to be
    swapped when porting. This incompatibility is unfortunate but is
    unlikely to be changed. (See [ (Integer
    Variant)](#plpgsql-integer-for).)

-   `FOR` loops over queries (other than cursors) also work differently:
    the target variable(s) must have been declared, whereas PL/SQL
    always declares them implicitly. An advantage of this is that the
    variable values are still accessible after the loop exits.

-   There are various notational differences for the use of cursor
    variables.

Porting Examples
----------------

[example\_title](#pgsql-porting-ex1) shows how to port a simple function
from PL/SQL to PL/pgSQL.

Here is an Oracle PL/SQL function:

    CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar2,
                                                      v_version varchar2)
    RETURN varchar2 IS
    BEGIN
        IF v_version IS NULL THEN
            RETURN v_name;
        END IF;
        RETURN v_name || '/' || v_version;
    END;
    /
    show errors;

Let\'s go through this function and see the differences compared to
PL/pgSQL:

-   The type name `varchar2` has to be changed to `varchar` or `text`.
    In the examples in this section, we\'ll use `varchar`, but `text` is
    often a better choice if you do not need specific string length
    limits.

-   The `RETURN` key word in the function prototype (not the function
    body) becomes `RETURNS` in PostgreSQL. Also, `IS` becomes `AS`, and
    you need to add a `LANGUAGE` clause because PL/pgSQL is not the only
    possible function language.

-   In PostgreSQL, the function body is considered to be a string
    literal, so you need to use quote marks or dollar quotes around it.
    This substitutes for the terminating `/` in the Oracle approach.

-   The `show errors` command does not exist in PostgreSQL, and is not
    needed since errors are reported automatically.

This is how this function would look when ported to PostgreSQL:

    CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name varchar,
                                                      v_version varchar)
    RETURNS varchar AS $$
    BEGIN
        IF v_version IS NULL THEN
            RETURN v_name;
        END IF;
        RETURN v_name || '/' || v_version;
    END;
    $$ LANGUAGE plpgsql;

[example\_title](#plpgsql-porting-ex2) shows how to port a function that
creates another function and how to handle the ensuing quoting problems.

The following procedure grabs rows from a `SELECT` statement and builds
a large function with the results in `IF` statements, for the sake of
efficiency.

This is the Oracle version:

    CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
        CURSOR referrer_keys IS
            SELECT * FROM cs_referrer_keys
            ORDER BY try_order;
        func_cmd VARCHAR(4000);
    BEGIN
        func_cmd := 'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host IN VARCHAR2,
                     v_domain IN VARCHAR2, v_url IN VARCHAR2) RETURN VARCHAR2 IS BEGIN';

        FOR referrer_key IN referrer_keys LOOP
            func_cmd := func_cmd ||
              ' IF v_' || referrer_key.kind
              || ' LIKE ''' || referrer_key.key_string
              || ''' THEN RETURN ''' || referrer_key.referrer_type
              || '''; END IF;';
        END LOOP;

        func_cmd := func_cmd || ' RETURN NULL; END;';

        EXECUTE IMMEDIATE func_cmd;
    END;
    /
    show errors;

Here is how this function would end up in PostgreSQL:

    CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc() AS $func$
    DECLARE
        referrer_keys CURSOR IS
            SELECT * FROM cs_referrer_keys
            ORDER BY try_order;
        func_body text;
        func_cmd text;
    BEGIN
        func_body := 'BEGIN';

        FOR referrer_key IN referrer_keys LOOP
            func_body := func_body ||
              ' IF v_' || referrer_key.kind
              || ' LIKE ' || quote_literal(referrer_key.key_string)
              || ' THEN RETURN ' || quote_literal(referrer_key.referrer_type)
              || '; END IF;' ;
        END LOOP;

        func_body := func_body || ' RETURN NULL; END;';

        func_cmd :=
          'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host varchar,
                                                            v_domain varchar,
                                                            v_url varchar)
            RETURNS varchar AS '
          || quote_literal(func_body)
          || ' LANGUAGE plpgsql;' ;

        EXECUTE func_cmd;
    END;
    $func$ LANGUAGE plpgsql;

Notice how the body of the function is built separately and passed
through `quote_literal` to double any quote marks in it. This technique
is needed because we cannot safely use dollar quoting for defining the
new function: we do not know for sure what strings will be interpolated
from the referrer\_key.key\_string field. (We are assuming here that
referrer\_key.kind can be trusted to always be `host`, `domain`, or
`url`, but referrer\_key.key\_string might be anything, in particular it
might contain dollar signs.) This function is actually an improvement on
the Oracle original, because it will not generate broken code when
referrer\_key.key\_string or referrer\_key.referrer\_type contain quote
marks.

[example\_title](#plpgsql-porting-ex3) shows how to port a function with
`OUT` parameters and string manipulation. PostgreSQL does not have a
built-in `instr` function, but you can create one using a combination of
other functions. In [Appendix](#plpgsql-porting-appendix) there is a
PL/pgSQL implementation of `instr` that you can use to make your porting
easier.

The following Oracle PL/SQL procedure is used to parse a URL and return
several elements (host, path, and query).

This is the Oracle version:

    CREATE OR REPLACE PROCEDURE cs_parse_url(
        v_url IN VARCHAR2,
        v_host OUT VARCHAR2,  -- This will be passed back
        v_path OUT VARCHAR2,  -- This one too
        v_query OUT VARCHAR2) -- And this one
    IS
        a_pos1 INTEGER;
        a_pos2 INTEGER;
    BEGIN
        v_host := NULL;
        v_path := NULL;
        v_query := NULL;
        a_pos1 := instr(v_url, '//');

        IF a_pos1 = 0 THEN
            RETURN;
        END IF;
        a_pos2 := instr(v_url, '/', a_pos1 + 2);
        IF a_pos2 = 0 THEN
            v_host := substr(v_url, a_pos1 + 2);
            v_path := '/';
            RETURN;
        END IF;

        v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
        a_pos1 := instr(v_url, '?', a_pos2 + 1);

        IF a_pos1 = 0 THEN
            v_path := substr(v_url, a_pos2);
            RETURN;
        END IF;

        v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
        v_query := substr(v_url, a_pos1 + 1);
    END;
    /
    show errors;

Here is a possible translation into PL/pgSQL:

    CREATE OR REPLACE FUNCTION cs_parse_url(
        v_url IN VARCHAR,
        v_host OUT VARCHAR,  -- This will be passed back
        v_path OUT VARCHAR,  -- This one too
        v_query OUT VARCHAR) -- And this one
    AS $$
    DECLARE
        a_pos1 INTEGER;
        a_pos2 INTEGER;
    BEGIN
        v_host := NULL;
        v_path := NULL;
        v_query := NULL;
        a_pos1 := instr(v_url, '//');

        IF a_pos1 = 0 THEN
            RETURN;
        END IF;
        a_pos2 := instr(v_url, '/', a_pos1 + 2);
        IF a_pos2 = 0 THEN
            v_host := substr(v_url, a_pos1 + 2);
            v_path := '/';
            RETURN;
        END IF;

        v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
        a_pos1 := instr(v_url, '?', a_pos2 + 1);

        IF a_pos1 = 0 THEN
            v_path := substr(v_url, a_pos2);
            RETURN;
        END IF;

        v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
        v_query := substr(v_url, a_pos1 + 1);
    END;
    $$ LANGUAGE plpgsql;

This function could be used like this:

    SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');

[example\_title](#plpgsql-porting-ex4) shows how to port a procedure
that uses numerous features that are specific to Oracle.

The Oracle version:

    CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
        a_running_job_count INTEGER;
    BEGIN
        LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

        SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

        IF a_running_job_count > 0 THEN
            COMMIT; -- free lock
            raise_application_error(-20000,
                     'Unable to create a new job: a job is currently running.');
        END IF;

        DELETE FROM cs_active_job;
        INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

        BEGIN
            INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
        EXCEPTION
            WHEN dup_val_on_index THEN NULL; -- don't worry if it already exists
        END;
        COMMIT;
    END;
    /
    show errors

This is how we could port this procedure to PL/pgSQL:

    CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id integer) AS $$
    DECLARE
        a_running_job_count integer;
    BEGIN
        LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

        SELECT count(*) INTO a_running_job_count FROM cs_jobs WHERE end_stamp IS NULL;

        IF a_running_job_count > 0 THEN
            COMMIT; -- free lock
            RAISE EXCEPTION 'Unable to create a new job: a job is currently running'; -- 
        END IF;

        DELETE FROM cs_active_job;
        INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

        BEGIN
            INSERT INTO cs_jobs (job_id, start_stamp) VALUES (v_job_id, now());
        EXCEPTION
            WHEN unique_violation THEN -- 
                -- don't worry if it already exists
        END;
        COMMIT;
    END;
    $$ LANGUAGE plpgsql;

-   The syntax of `RAISE` is considerably different from Oracle\'s
    statement, although the basic case `RAISE` exception\_name works
    similarly.

-   The exception names supported by PL/pgSQL are different from
    Oracle\'s. The set of built-in exception names is much larger (see
    [???](#errcodes-appendix)). There is not currently a way to declare
    user-defined exception names, although you can throw user-chosen
    SQLSTATE values instead.

Other Things to Watch For {#plpgsql-porting-other}
-------------------------

This section explains a few other things to watch for when porting
Oracle PL/SQL functions to PostgreSQL.

### Implicit Rollback after Exceptions {#plpgsql-porting-exceptions}

In PL/pgSQL, when an exception is caught by an `EXCEPTION` clause, all
database changes since the block\'s `BEGIN` are automatically rolled
back. That is, the behavior is equivalent to what you\'d get in Oracle
with:

    BEGIN
        SAVEPOINT s1;
        ... code here ...
    EXCEPTION
        WHEN ... THEN
            ROLLBACK TO s1;
            ... code here ...
        WHEN ... THEN
            ROLLBACK TO s1;
            ... code here ...
    END;

If you are translating an Oracle procedure that uses `SAVEPOINT` and
`ROLLBACK TO` in this style, your task is easy: just omit the
`SAVEPOINT` and `ROLLBACK TO`. If you have a procedure that uses
`SAVEPOINT` and `ROLLBACK TO` in a different way then some actual
thought will be required.

### `EXECUTE`

The PL/pgSQL version of `EXECUTE` works similarly to the PL/SQL version,
but you have to remember to use `quote_literal` and `quote_ident` as
described in [Executing Dynamic
Commands](#plpgsql-statements-executing-dyn). Constructs of the type
`EXECUTE 'SELECT * FROM $1';` will not work reliably unless you use
these functions.

### Optimizing PL/pgSQL Functions {#plpgsql-porting-optimization}

PostgreSQL gives you two function creation modifiers to optimize
execution: "volatility" (whether the function always returns the same
result when given the same arguments) and "strictness" (whether the
function returns null if any argument is null). Consult the
[???](#sql-createfunction) reference page for details.

When making use of these optimization attributes, your `CREATE FUNCTION`
statement might look something like this:

    CREATE FUNCTION foo(...) RETURNS integer AS $$
    ...
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;

Appendix {#plpgsql-porting-appendix}
--------

This section contains the code for a set of Oracle-compatible `instr`
functions that you can use to simplify your porting efforts.

instr

function

    --
    -- instr functions that mimic Oracle's counterpart
    -- Syntax: instr(string1, string2 [, n [, m]])
    -- where [] denotes optional parameters.
    --
    -- Search string1, beginning at the nth character, for the mth occurrence
    -- of string2.  If n is negative, search backwards, starting at the abs(n)'th
    -- character from the end of string1.
    -- If n is not passed, assume 1 (search starts at first character).
    -- If m is not passed, assume 1 (find first occurrence).
    -- Returns starting index of string2 in string1, or 0 if string2 is not found.
    --

    CREATE FUNCTION instr(varchar, varchar) RETURNS integer AS $$
    BEGIN
        RETURN instr($1, $2, 1);
    END;
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;


    CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
                          beg_index integer)
    RETURNS integer AS $$
    DECLARE
        pos integer NOT NULL DEFAULT 0;
        temp_str varchar;
        beg integer;
        length integer;
        ss_length integer;
    BEGIN
        IF beg_index > 0 THEN
            temp_str := substring(string FROM beg_index);
            pos := position(string_to_search_for IN temp_str);

            IF pos = 0 THEN
                RETURN 0;
            ELSE
                RETURN pos + beg_index - 1;
            END IF;
        ELSIF beg_index < 0 THEN
            ss_length := char_length(string_to_search_for);
            length := char_length(string);
            beg := length + 1 + beg_index;

            WHILE beg > 0 LOOP
                temp_str := substring(string FROM beg FOR ss_length);
                IF string_to_search_for = temp_str THEN
                    RETURN beg;
                END IF;

                beg := beg - 1;
            END LOOP;

            RETURN 0;
        ELSE
            RETURN 0;
        END IF;
    END;
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;


    CREATE FUNCTION instr(string varchar, string_to_search_for varchar,
                          beg_index integer, occur_index integer)
    RETURNS integer AS $$
    DECLARE
        pos integer NOT NULL DEFAULT 0;
        occur_number integer NOT NULL DEFAULT 0;
        temp_str varchar;
        beg integer;
        i integer;
        length integer;
        ss_length integer;
    BEGIN
        IF occur_index <= 0 THEN
            RAISE 'argument ''%'' is out of range', occur_index
              USING ERRCODE = '22003';
        END IF;

        IF beg_index > 0 THEN
            beg := beg_index - 1;
            FOR i IN 1..occur_index LOOP
                temp_str := substring(string FROM beg + 1);
                pos := position(string_to_search_for IN temp_str);
                IF pos = 0 THEN
                    RETURN 0;
                END IF;
                beg := beg + pos;
            END LOOP;

            RETURN beg;
        ELSIF beg_index < 0 THEN
            ss_length := char_length(string_to_search_for);
            length := char_length(string);
            beg := length + 1 + beg_index;

            WHILE beg > 0 LOOP
                temp_str := substring(string FROM beg FOR ss_length);
                IF string_to_search_for = temp_str THEN
                    occur_number := occur_number + 1;
                    IF occur_number = occur_index THEN
                        RETURN beg;
                    END IF;
                END IF;

                beg := beg - 1;
            END LOOP;

            RETURN 0;
        ELSE
            RETURN 0;
        END IF;
    END;
    $$ LANGUAGE plpgsql STRICT IMMUTABLE;
