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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,
       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 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. Prepared statements 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

       If a prepared statement is executed enough times, the server may
       eventually decide to save and re-use a generic plan rather than
       re-planning each time. This will occur immediately if the prepared
       statement has no parameters; otherwise it occurs only if the generic
       plan appears to be not much more expensive than a plan that depends on
       specific parameter values. Typically, a generic plan will be selected
       only if the query's performance is estimated to be fairly insensitive
       to the specific parameter values supplied.

       To examine the query plan PostgreSQL is using for a prepared statement,
       use EXPLAIN(7). If a generic plan is in use, it will contain parameter
       symbols $n, while a custom plan will have the current actual parameter
       values substituted into it.

       For more information on query planning and the statistics collected by
       PostgreSQL for that purpose, see the ANALYZE(7) 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 since the previous use of the prepared statement. Also,
       if the value of 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 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)



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