CREATE FUNCTION

CREATE FUNCTION

7

SQL - Language Statements

CREATE FUNCTION

define a new function

CREATE \[ OR REPLACE \] FUNCTION

name

( \[ \[

argmode

\] \[

argname

\]

argtype

\[ { DEFAULT \| = }

default\_expr

\] \[, \...\] \] ) \[ RETURNS

rettype

\| RETURNS TABLE (

column\_name

column\_type

\[, \...\] ) \] { LANGUAGE

lang\_name

\| TRANSFORM { FOR TYPE

type\_name

} \[, \... \] \| WINDOW \| { IMMUTABLE \| STABLE \| VOLATILE } \| \[ NOT
\] LEAKPROOF \| { CALLED ON NULL INPUT \| RETURNS NULL ON NULL INPUT \|
STRICT } \| { \[ EXTERNAL \] SECURITY INVOKER \| \[ EXTERNAL \] SECURITY
DEFINER } \| PARALLEL { UNSAFE \| RESTRICTED \| SAFE } \| COST

execution\_cost

\| ROWS

result\_rows

\| SUPPORT

support\_function

\| SET

configuration\_parameter

{ TO

value

\| =

value

\| FROM CURRENT } \| AS \'

definition

\' \| AS \'

obj\_file

\', \'

link\_symbol

\' \|

sql\_body

} \...

Description {#sql-createfunction-description}
===========

`CREATE FUNCTION` defines a new function. `CREATE OR REPLACE FUNCTION`
will either create a new function, or replace an existing definition. To
be able to define a function, the user must have the `USAGE` privilege
on the language.

If a schema name is included, then the function is created in the
specified schema. Otherwise it is created in the current schema. The
name of the new function must not match any existing function or
procedure with the same input argument types in the same schema.
However, functions and procedures of different argument types can share
a name (this is called overloading).

To replace the current definition of an existing function, use
`CREATE OR REPLACE FUNCTION`. It is not possible to change the name or
argument types of a function this way (if you tried, you would actually
be creating a new, distinct function). Also,
`CREATE OR REPLACE FUNCTION` will not let you change the return type of
an existing function. To do that, you must drop and recreate the
function. (When using `OUT` parameters, that means you cannot change the
types of any `OUT` parameters except by dropping the function.)

When `CREATE OR REPLACE FUNCTION` is used to replace an existing
function, the ownership and permissions of the function do not change.
All other function properties are assigned the values specified or
implied in the command. You must own the function to replace it (this
includes being a member of the owning role).

If you drop and then recreate a function, the new function is not the
same entity as the old; you will have to drop existing rules, views,
triggers, etc. that refer to the old function. Use
`CREATE OR REPLACE FUNCTION` to change a function definition without
breaking objects that refer to the function. Also, `ALTER FUNCTION` can
be used to change most of the auxiliary properties of an existing
function.

The user that creates the function becomes the owner of the function.

To be able to create a function, you must have `USAGE` privilege on the
argument types and the return type.

Refer to [???](#xfunc) for further information on writing functions.

Parameters
==========

name

:   The name (optionally schema-qualified) of the function to create.

argmode

:   The mode of an argument: `IN`, `OUT`, `INOUT`, or `VARIADIC`. If
    omitted, the default is `IN`. Only `OUT` arguments can follow a
    `VARIADIC` one. Also, `OUT` and `INOUT` arguments cannot be used
    together with the `RETURNS TABLE` notation.

argname

:   The name of an argument. Some languages (including SQL and PL/pgSQL)
    let you use the name in the function body. For other languages the
    name of an input argument is just extra documentation, so far as the
    function itself is concerned; but you can use input argument names
    when calling a function to improve readability (see
    [???](#sql-syntax-calling-funcs)). In any case, the name of an
    output argument is significant, because it defines the column name
    in the result row type. (If you omit the name for an output
    argument, the system will choose a default column name.)

argtype

:   The data type(s) of the function\'s arguments (optionally
    schema-qualified), if any. The argument types can be base,
    composite, or domain types, or can reference the type of a table
    column.

    Depending on the implementation language it might also be allowed to
    specify "pseudo-types" such as `cstring`. Pseudo-types indicate that
    the actual argument type is either incompletely specified, or
    outside the set of ordinary SQL data types.

    The type of a column is referenced by writing
    `table_name.column_name%TYPE`. Using this feature can sometimes help
    make a function independent of changes to the definition of a table.

default\_expr

:   An expression to be used as default value if the parameter is not
    specified. The expression has to be coercible to the argument type
    of the parameter. Only input (including `INOUT`) parameters can have
    a default value. All input parameters following a parameter with a
    default value must have default values as well.

rettype

:   The return data type (optionally schema-qualified). The return type
    can be a base, composite, or domain type, or can reference the type
    of a table column. Depending on the implementation language it might
    also be allowed to specify "pseudo-types" such as `cstring`. If the
    function is not supposed to return a value, specify `void` as the
    return type.

    When there are `OUT` or `INOUT` parameters, the `RETURNS` clause can
    be omitted. If present, it must agree with the result type implied
    by the output parameters: `RECORD` if there are multiple output
    parameters, or the same type as the single output parameter.

    The `SETOF` modifier indicates that the function will return a set
    of items, rather than a single item.

    The type of a column is referenced by writing
    `table_name.column_name%TYPE`.

column\_name

:   The name of an output column in the `RETURNS TABLE` syntax. This is
    effectively another way of declaring a named `OUT` parameter, except
    that `RETURNS TABLE` also implies `RETURNS SETOF`.

column\_type

:   The data type of an output column in the `RETURNS TABLE` syntax.

lang\_name

:   The name of the language that the function is implemented in. It can
    be `sql`, `c`, `internal`, or the name of a user-defined procedural
    language, e.g., `plpgsql`. The default is `sql` if sql\_body is
    specified. Enclosing the name in single quotes is deprecated and
    requires matching case.

`TRANSFORM { FOR TYPE type_name } [, ... ] }`

:   Lists which transforms a call to the function should apply.
    Transforms convert between SQL types and language-specific data
    types; see [???](#sql-createtransform). Procedural language
    implementations usually have hardcoded knowledge of the built-in
    types, so those don\'t need to be listed here. If a procedural
    language implementation does not know how to handle a type and no
    transform is supplied, it will fall back to a default behavior for
    converting data types, but this depends on the implementation.

`WINDOW`

:   `WINDOW` indicates that the function is a window function rather
    than a plain function. This is currently only useful for functions
    written in C. The `WINDOW` attribute cannot be changed when
    replacing an existing function definition.

`IMMUTABLE`; `STABLE`; `VOLATILE`

:   These attributes inform the query optimizer about the behavior of
    the function. At most one choice can be specified. If none of these
    appear, `VOLATILE` is the default assumption.

    `IMMUTABLE` indicates that the function cannot modify the database
    and always returns the same result when given the same argument
    values; that is, it does not do database lookups or otherwise use
    information not directly present in its argument list. If this
    option is given, any call of the function with all-constant
    arguments can be immediately replaced with the function value.

    `STABLE` indicates that the function cannot modify the database, and
    that within a single table scan it will consistently return the same
    result for the same argument values, but that its result could
    change across SQL statements. This is the appropriate selection for
    functions whose results depend on database lookups, parameter
    variables (such as the current time zone), etc. (It is inappropriate
    for `AFTER` triggers that wish to query rows modified by the current
    command.) Also note that the `current_timestamp` family of functions
    qualify as stable, since their values do not change within a
    transaction.

    `VOLATILE` indicates that the function value can change even within
    a single table scan, so no optimizations can be made. Relatively few
    database functions are volatile in this sense; some examples are
    `random()`, `currval()`, `timeofday()`. But note that any function
    that has side-effects must be classified volatile, even if its
    result is quite predictable, to prevent calls from being optimized
    away; an example is `setval()`.

    For additional details see [???](#xfunc-volatility).

`LEAKPROOF`

:   `LEAKPROOF` indicates that the function has no side effects. It
    reveals no information about its arguments other than by its return
    value. For example, a function which throws an error message for
    some argument values but not others, or which includes the argument
    values in any error message, is not leakproof. This affects how the
    system executes queries against views created with the
    `security_barrier` option or tables with row level security enabled.
    The system will enforce conditions from security policies and
    security barrier views before any user-supplied conditions from the
    query itself that contain non-leakproof functions, in order to
    prevent the inadvertent exposure of data. Functions and operators
    marked as leakproof are assumed to be trustworthy, and may be
    executed before conditions from security policies and security
    barrier views. In addition, functions which do not take arguments or
    which are not passed any arguments from the security barrier view or
    table do not have to be marked as leakproof to be executed before
    security conditions. See [???](#sql-createview) and
    [???](#rules-privileges). This option can only be set by the
    superuser.

`CALLED ON NULL INPUT`; `RETURNS NULL ON NULL INPUT`; `STRICT`

:   `CALLED ON NULL INPUT` (the default) indicates that the function
    will be called normally when some of its arguments are null. It is
    then the function author\'s responsibility to check for null values
    if necessary and respond appropriately.

    `RETURNS NULL ON NULL INPUT` or `STRICT` indicates that the function
    always returns null whenever any of its arguments are null. If this
    parameter is specified, the function is not executed when there are
    null arguments; instead a null result is assumed automatically.

`EXTERNAL SECURITY INVOKER`; `EXTERNAL SECURITY DEFINER`

:   `SECURITY INVOKER` indicates that the function is to be executed
    with the privileges of the user that calls it. That is the default.
    `SECURITY DEFINER` specifies that the function is to be executed
    with the privileges of the user that owns it.

    The key word `EXTERNAL` is allowed for SQL conformance, but it is
    optional since, unlike in SQL, this feature applies to all functions
    not only external ones.

`PARALLEL`

:   `PARALLEL UNSAFE` indicates that the function can\'t be executed in
    parallel mode and the presence of such a function in an SQL
    statement forces a serial execution plan. This is the default.
    `PARALLEL RESTRICTED` indicates that the function can be executed in
    parallel mode, but the execution is restricted to parallel group
    leader. `PARALLEL SAFE` indicates that the function is safe to run
    in parallel mode without restriction.

    Functions should be labeled parallel unsafe if they modify any
    database state, or if they make changes to the transaction such as
    using sub-transactions, or if they access sequences or attempt to
    make persistent changes to settings (e.g., `setval`). They should be
    labeled as parallel restricted if they access temporary tables,
    client connection state, cursors, prepared statements, or
    miscellaneous backend-local state which the system cannot
    synchronize in parallel mode (e.g., `setseed` cannot be executed
    other than by the group leader because a change made by another
    process would not be reflected in the leader). In general, if a
    function is labeled as being safe when it is restricted or unsafe,
    or if it is labeled as being restricted when it is in fact unsafe,
    it may throw errors or produce wrong answers when used in a parallel
    query. C-language functions could in theory exhibit totally
    undefined behavior if mislabeled, since there is no way for the
    system to protect itself against arbitrary C code, but in most
    likely cases the result will be no worse than for any other
    function. If in doubt, functions should be labeled as `UNSAFE`,
    which is the default.

`COST` execution\_cost

:   A positive number giving the estimated execution cost for the
    function, in units of [???](#guc-cpu-operator-cost). If the function
    returns a set, this is the cost per returned row. If the cost is not
    specified, 1 unit is assumed for C-language and internal functions,
    and 100 units for functions in all other languages. Larger values
    cause the planner to try to avoid evaluating the function more often
    than necessary.

`ROWS` result\_rows

:   A positive number giving the estimated number of rows that the
    planner should expect the function to return. This is only allowed
    when the function is declared to return a set. The default
    assumption is 1000 rows.

`SUPPORT` support\_function

:   The name (optionally schema-qualified) of a planner support function
    to use for this function. See [???](#xfunc-optimization) for
    details. You must be superuser to use this option.

configuration\_parameter; value

:   The `SET` clause causes the specified configuration parameter to be
    set to the specified value when the function is entered, and then
    restored to its prior value when the function exits.
    `SET FROM CURRENT` saves the value of the parameter that is current
    when `CREATE FUNCTION` is executed as the value to be applied when
    the function is entered.

    If a `SET` clause is attached to a function, then the effects of a
    `SET LOCAL` command executed inside the function for the same
    variable are restricted to the function: the configuration
    parameter\'s prior value is still restored at function exit.
    However, an ordinary `SET` command (without `LOCAL`) overrides the
    `SET` clause, much as it would do for a previous `SET
           LOCAL` command: the effects of such a command will persist
    after function exit, unless the current transaction is rolled back.

    See [???](#sql-set) and [???](#runtime-config) for more information
    about allowed parameter names and values.

definition

:   A string constant defining the function; the meaning depends on the
    language. It can be an internal function name, the path to an object
    file, an SQL command, or text in a procedural language.

    It is often helpful to use dollar quoting (see
    [???](#sql-syntax-dollar-quoting)) to write the function definition
    string, rather than the normal single quote syntax. Without dollar
    quoting, any single quotes or backslashes in the function definition
    must be escaped by doubling them.

`obj_file, link_symbol`

:   This form of the `AS` clause is used for dynamically loadable C
    language functions when the function name in the C language source
    code is not the same as the name of the SQL function. The string
    obj\_file is the name of the shared library file containing the
    compiled C function, and is interpreted as for the
    [`LOAD`](#sql-load) command. The string link\_symbol is the
    function\'s link symbol, that is, the name of the function in the C
    language source code. If the link symbol is omitted, it is assumed
    to be the same as the name of the SQL function being defined. The C
    names of all functions must be different, so you must give
    overloaded C functions different C names (for example, use the
    argument types as part of the C names).

    When repeated `CREATE FUNCTION` calls refer to the same object file,
    the file is only loaded once per session. To unload and reload the
    file (perhaps during development), start a new session.

sql\_body

:   The body of a `LANGUAGE SQL` function. This can either be a single
    statement

        RETURN expression

    or a block

        BEGIN ATOMIC
          statement;
          statement;
          ...
          statement;
        END

    This is similar to writing the text of the function body as a string
    constant (see definition above), but there are some differences:
    This form only works for `LANGUAGE
           SQL`, the string constant form works for all languages. This
    form is parsed at function definition time, the string constant form
    is parsed at execution time; therefore this form cannot support
    polymorphic argument types and other constructs that are not
    resolvable at function definition time. This form tracks
    dependencies between the function and objects used in the function
    body, so `DROP
           ... CASCADE` will work correctly, whereas the form using
    string literals may leave dangling functions. Finally, this form is
    more compatible with the SQL standard and other SQL implementations.

Overloading {#sql-createfunction-overloading}
===========

PostgreSQL allows function overloading; that is, the same name can be
used for several different functions so long as they have distinct input
argument types. Whether or not you use it, this capability entails
security precautions when calling functions in databases where some
users mistrust other users; see [???](#typeconv-func).

Two functions are considered the same if they have the same names and
*input* argument types, ignoring any `OUT` parameters. Thus for example
these declarations conflict:

    CREATE FUNCTION foo(int) ...
    CREATE FUNCTION foo(int, out text) ...

Functions that have different argument type lists will not be considered
to conflict at creation time, but if defaults are provided they might
conflict in use. For example, consider

    CREATE FUNCTION foo(int) ...
    CREATE FUNCTION foo(int, int default 42) ...

A call `foo(10)` will fail due to the ambiguity about which function
should be called.

Notes {#sql-createfunction-notes}
=====

The full SQL type syntax is allowed for declaring a function\'s
arguments and return value. However, parenthesized type modifiers (e.g.,
the precision field for type `numeric`) are discarded by
`CREATE FUNCTION`. Thus for example
`CREATE FUNCTION foo (varchar(10)) ...` is exactly the same as
`CREATE FUNCTION foo (varchar) ...`.

When replacing an existing function with `CREATE OR REPLACE
    FUNCTION`, there are restrictions on changing parameter names. You
cannot change the name already assigned to any input parameter (although
you can add names to parameters that had none before). If there is more
than one output parameter, you cannot change the names of the output
parameters, because that would change the column names of the anonymous
composite type that describes the function\'s result. These restrictions
are made to ensure that existing calls of the function do not stop
working when it is replaced.

If a function is declared `STRICT` with a `VARIADIC` argument, the
strictness check tests that the variadic array *as a whole* is non-null.
The function will still be called if the array has null elements.

Examples {#sql-createfunction-examples}
========

Add two integers using an SQL function:

    CREATE FUNCTION add(integer, integer) RETURNS integer
        AS 'select $1 + $2;'
        LANGUAGE SQL
        IMMUTABLE
        RETURNS NULL ON NULL INPUT;

The same function written in a more SQL-conforming style, using argument
names and an unquoted body:

    CREATE FUNCTION add(a integer, b integer) RETURNS integer
        LANGUAGE SQL
        IMMUTABLE
        RETURNS NULL ON NULL INPUT
        RETURN a + b;

Increment an integer, making use of an argument name, in PL/pgSQL:

    CREATE OR REPLACE FUNCTION increment(i integer) RETURNS integer AS $$
            BEGIN
                    RETURN i + 1;
            END;
    $$ LANGUAGE plpgsql;

Return a record containing multiple output parameters:

    CREATE FUNCTION dup(in int, out f1 int, out f2 text)
        AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
        LANGUAGE SQL;

    SELECT * FROM dup(42);

You can do the same thing more verbosely with an explicitly named
composite type:

    CREATE TYPE dup_result AS (f1 int, f2 text);

    CREATE FUNCTION dup(int) RETURNS dup_result
        AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
        LANGUAGE SQL;

    SELECT * FROM dup(42);

Another way to return multiple columns is to use a `TABLE` function:

    CREATE FUNCTION dup(int) RETURNS TABLE(f1 int, f2 text)
        AS $$ SELECT $1, CAST($1 AS text) || ' is text' $$
        LANGUAGE SQL;

    SELECT * FROM dup(42);

However, a `TABLE` function is different from the preceding examples,
because it actually returns a *set* of records, not just one record.

Writing `SECURITY DEFINER` Functions Safely {#sql-createfunction-security}
===========================================

search\_path

configuration parameter

use in securing functions

Because a `SECURITY DEFINER` function is executed with the privileges of
the user that owns it, care is needed to ensure that the function cannot
be misused. For security, [???](#guc-search-path) should be set to
exclude any schemas writable by untrusted users. This prevents malicious
users from creating objects (e.g., tables, functions, and operators)
that mask objects intended to be used by the function. Particularly
important in this regard is the temporary-table schema, which is
searched first by default, and is normally writable by anyone. A secure
arrangement can be obtained by forcing the temporary schema to be
searched last. To do this, write `pg_temp`pg\_tempsecuring functions as
the last entry in `search_path`. This function illustrates safe usage:

    CREATE FUNCTION check_password(uname TEXT, pass TEXT)
    RETURNS BOOLEAN AS $$
    DECLARE passed BOOLEAN;
    BEGIN
            SELECT  (pwd = $2) INTO passed
            FROM    pwds
            WHERE   username = $1;

            RETURN passed;
    END;
    $$  LANGUAGE plpgsql
        SECURITY DEFINER
        -- Set a secure search_path: trusted schema(s), then 'pg_temp'.
        SET search_path = admin, pg_temp;

This function\'s intention is to access a table `admin.pwds`. But
without the `SET` clause, or with a `SET` clause mentioning only
`admin`, the function could be subverted by creating a temporary table
named `pwds`.

Before PostgreSQL version 8.3, the `SET` clause was not available, and
so older functions may contain rather complicated logic to save, set,
and restore `search_path`. The `SET` clause is far easier to use for
this purpose.

Another point to keep in mind is that by default, execute privilege is
granted to `PUBLIC` for newly created functions (see [???](#ddl-priv)
for more information). Frequently you will wish to restrict use of a
security definer function to only some users. To do that, you must
revoke the default `PUBLIC` privileges and then grant execute privilege
selectively. To avoid having a window where the new function is
accessible to all, create it and set the privileges within a single
transaction. For example:

    BEGIN;
    CREATE FUNCTION check_password(uname TEXT, pass TEXT) ... SECURITY DEFINER;
    REVOKE ALL ON FUNCTION check_password(uname TEXT, pass TEXT) FROM PUBLIC;
    GRANT EXECUTE ON FUNCTION check_password(uname TEXT, pass TEXT) TO admins;
    COMMIT;

Compatibility {#sql-createfunction-compat}
=============

A `CREATE FUNCTION` command is defined in the SQL standard. The
PostgreSQL implementation can be used in a compatible way but has many
extensions. Conversely, the SQL standard specifies a number of optional
features that are not implemented in PostgreSQL.

The following are important compatibility issues:

-   `OR REPLACE` is a PostgreSQL extension.

-   For compatibility with some other database systems, argmode can be
    written either before or after argname. But only the first way is
    standard-compliant.

-   For parameter defaults, the SQL standard specifies only the syntax
    with the `DEFAULT` key word. The syntax with `=` is used in T-SQL
    and Firebird.

-   The `SETOF` modifier is a PostgreSQL extension.

-   Only `SQL` is standardized as a language.

-   All other attributes except `CALLED ON NULL INPUT` and
    `RETURNS NULL ON NULL INPUT` are not standardized.

-   For the body of `LANGUAGE SQL` functions, the SQL standard only
    specifies the sql\_body form.

Simple `LANGUAGE SQL` functions can be written in a way that is both
standard-conforming and portable to other implementations. More complex
functions using advanced features, optimization attributes, or other
languages will necessarily be specific to PostgreSQL in a significant
way.

See Also
========
