Provided by: postgresql-client-9.1_9.1.1-1_i386 bug

NAME

       PREPARE - prepare a statement for execution

SYNOPSIS

       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,
       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 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 used (if possible). When executing the statement, specify the actual
       values for these parameters in the EXECUTE statement. Refer to
       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(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 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 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 planning prepared
       statements with parameters, the chosen plan might be suboptimal. To
       examine the query plan PostgreSQL has chosen for a prepared statement,
       use EXPLAIN(7).

       For more information on query planning and the statistics collected by
       PostgreSQL for that purpose, see the ANALYZE(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 statement also uses a
       somewhat different syntax.

SEE ALSO

       DEALLOCATE(7), EXECUTE(7)