PREPARE

prepared statements

creating

PREPARE

7

SQL - Language Statements

PREPARE

prepare a statement for execution

PREPARE

name

\[ (

data\_type

\[, \...\] ) \] AS

statement

Description
===========

`PREPARE` creates a prepared statement. A prepared statement is a
server-side object that can be used to optimize performance. When the
`PREPARE` statement is executed, the specified statement is parsed,
analyzed, and rewritten. When an `EXECUTE` command is subsequently
issued, the prepared statement is planned and executed. This division of
labor avoids repetitive parse analysis work, while allowing the
execution plan to depend on the specific parameter values supplied.

Prepared statements can take parameters: values that are substituted
into the statement when it is executed. When creating the prepared
statement, refer to parameters by position, using `$1`, `$2`, etc. A
corresponding list of parameter data types can optionally be specified.
When a parameter\'s data type is not specified or is declared as
`unknown`, the type is inferred from the context in which the parameter
is first referenced (if possible). When executing the statement, specify
the actual values for these parameters in the `EXECUTE` statement. Refer
to [???](#sql-execute) for more information about that.

Prepared statements only last for the duration of the current database
session. When the session ends, the prepared statement is forgotten, so
it must be recreated before being used again. This also means that a
single prepared statement cannot be used by multiple simultaneous
database clients; however, each client can create their own prepared
statement to use. Prepared statements can be manually cleaned up using
the [`DEALLOCATE`](#sql-deallocate) command.

Prepared statements potentially have the largest performance advantage
when a single session is being used to execute a large number of similar
statements. The performance difference will be particularly significant
if the statements are complex to plan or rewrite, e.g., if the query
involves a join of many tables or requires the application of several
rules. If the statement is relatively simple to plan and rewrite but
relatively expensive to execute, the performance advantage of prepared
statements will be less noticeable.

Parameters
==========

name

:   An arbitrary name given to this particular prepared statement. It
    must be unique within a single session and is subsequently used to
    execute or deallocate a previously prepared statement.

data\_type

:   The data type of a parameter to the prepared statement. If the data
    type of a particular parameter is unspecified or is specified as
    `unknown`, it will be inferred from the context in which the
    parameter is first referenced. To refer to the parameters in the
    prepared statement itself, use `$1`, `$2`, etc.

statement

:   Any `SELECT`, `INSERT`, `UPDATE`, `DELETE`, or `VALUES` statement.

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

A prepared statement can be executed with either a generic plan or a
custom plan. A generic plan is the same across all executions, while a
custom plan is generated for a specific execution using the parameter
values given in that call. Use of a generic plan avoids planning
overhead, but in some situations a custom plan will be much more
efficient to execute because the planner can make use of knowledge of
the parameter values. (Of course, if the prepared statement has no
parameters, then this is moot and a generic plan is always used.)

By default (that is, when [???](#guc-plan-cache_mode) is set to `auto`),
the server will automatically choose whether to use a generic or custom
plan for a prepared statement that has parameters. The current rule for
this is that the first five executions are done with custom plans and
the average estimated cost of those plans is calculated. Then a generic
plan is created and its estimated cost is compared to the average
custom-plan cost. Subsequent executions use the generic plan if its cost
is not so much higher than the average custom-plan cost as to make
repeated replanning seem preferable.

This heuristic can be overridden, forcing the server to use either
generic or custom plans, by setting `plan_cache_mode` to
`force_generic_plan` or `force_custom_plan` respectively. This setting
is primarily useful if the generic plan\'s cost estimate is badly off
for some reason, allowing it to be chosen even though its actual cost is
much more than that of a custom plan.

To examine the query plan PostgreSQL is using for a prepared statement,
use [`EXPLAIN`](#sql-explain), for example

    EXPLAIN EXECUTE name(parameter_values);

If a generic plan is in use, it will contain parameter symbols `$n`,
while a custom plan will have the supplied parameter values substituted
into it.

For more information on query planning and the statistics collected by
PostgreSQL for that purpose, see the [???](#sql-analyze) documentation.

Although the main point of a prepared statement is to avoid repeated
parse analysis and planning of the statement, PostgreSQL will force
re-analysis and re-planning of the statement before using it whenever
database objects used in the statement have undergone definitional (DDL)
changes or their planner statistics have been updated since the previous
use of the prepared statement. Also, if the value of
[???](#guc-search-path) changes from one use to the next, the statement
will be re-parsed using the new `search_path`. (This latter behavior is
new as of PostgreSQL 9.3.) These rules make use of a prepared statement
semantically almost equivalent to re-submitting the same query text over
and over, but with a performance benefit if no object definitions are
changed, especially if the best plan remains the same across uses. An
example of a case where the semantic equivalence is not perfect is that
if the statement refers to a table by an unqualified name, and then a
new table of the same name is created in a schema appearing earlier in
the `search_path`, no automatic re-parse will occur since no object used
in the statement changed. However, if some other change forces a
re-parse, the new table will be referenced in subsequent uses.

You can see all prepared statements available in the session by querying
the [pg\_prepared\_statements](#view-pg-prepared-statements) system
view.

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

Create a prepared statement for an `INSERT` statement, and then execute
it:

    PREPARE fooplan (int, text, bool, numeric) AS
        INSERT INTO foo VALUES($1, $2, $3, $4);
    EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);

Create a prepared statement for a `SELECT` statement, and then execute
it:

    PREPARE usrrptplan (int) AS
        SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid
        AND l.date = $2;
    EXECUTE usrrptplan(1, current_date);

In this example, the data type of the second parameter is not specified,
so it is inferred from the context in which `$2` is used.

Compatibility
=============

The SQL standard includes a `PREPARE` statement, but it is only for use
in embedded SQL. This version of the `PREPARE` statement also uses a
somewhat different syntax.

See Also
========
