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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 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. The prepared statement can be manually cleaned up using the DEALLOCATE command.

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 for a prepared statement will be inferior to the query plan that would have been chosen if the statement had been 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.

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

Examples

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

PREPARE usrrptplan (int, date) 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);

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

DEALLOCATE, EXECUTE

Comments


Jan. 12, 2006, 6:57 a.m.

The table name cannot be specified with a parameter.

For example, the following will fail:
PREPARE plan1 (text) as SELECT * FROM $1;

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