PostgreSQL 7.4.3 Documentation
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# PREPARE

## Name

PREPARE -- prepare a statement for execution

## Synopsis

    PREPARE plan_name [ (datatype [, ...] ) ] 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, rewritten, and planned. When an `EXECUTE` command is subsequently issued, the prepared statement need only be executed. Thus, the parsing, rewriting, and planning stages are only performed once, instead of every time the statement is executed.

Prepared statements can take parameters: values that are substituted into the statement when it is executed. To include parameters in a prepared statement, supply a list of data types in the `PREPARE` statement, and, in the statement to be prepared itself, refer to the parameters by position using `$1`, `$2`, etc. When executing the statement, specify the actual values for these parameters in the `EXECUTE` statement. Refer to EXECUTE for more information about that.

Prepared statements are only stored in and for the duration of the current database session. When the session ends, the prepared statement is forgotten, and 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 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, for example, 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

plan_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.

datatype

  The data type of a parameter to the prepared statement. To refer to the parameters in the prepared statement itself, use `$1`, `$2`, etc.

statement

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

## Notes

In some situations, the query plan produced by for a prepared statement may be inferior to the plan produced if the statement were submitted and executed normally. This is because when the statement is planned and the planner attempts to determine the optimal query plan, the actual values of any parameters specified in the statement are unavailable. PostgreSQL collects statistics on the distribution of data in the table, and can use constant values in a statement to make guesses about the likely result of executing the statement. Since this data is unavailable when planning prepared statements with parameters, the chosen plan may be suboptimal. To examine the query plan PostgreSQL has chosen for a prepared statement, use `EXPLAIN EXECUTE`.

For more information on query planning and the statistics collected by PostgreSQL for that purpose, see the ANALYZE documentation.

## 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.

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