PREPARE(7)                                                SQL Commands                                                PREPARE(7)



NAME
       PREPARE - prepare a statement for execution


SYNOPSIS
       PREPARE name [ ( datatype [, ...] ) ] AS statement


DESCRIPTION
       PREPARE  creates  a prepared statement. A prepared statement is a server-side object that can be used to optimize perfor-
       mance. 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. When  creat-
       ing  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 used (if possible). When executing the statement, specify the actual
       values for these parameters in the EXECUTE statement. Refer to EXECUTE [execute(7)] 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 [deallocate(7)] 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 pre-
       pared 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.

       datatype
              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 used. To refer to the
              parameters in the prepared statement itself, use $1, $2, etc.

       statement
              Any SELECT, INSERT, UPDATE, DELETE, or VALUES 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 plan-
       ning  prepared  statements with parameters, the chosen plan might be suboptimal. To examine the query plan PostgreSQL has
       chosen for a prepared statement, use EXPLAIN [explain(7)].

       For more information on query planning and the statistics collected by PostgreSQL for that purpose, see the ANALYZE [ana-
       lyze(7)] documentation.

       You can see all available prepared statements of a session by querying the pg_prepared_statements system view.

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);

       Note that 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 state-
       ment also uses a somewhat different syntax.

SEE ALSO
       DEALLOCATE [deallocate(7)], EXECUTE [execute(7)]



SQL - Language Statements                                  2011-09-22                                                 PREPARE(7)