xenial (7) PREPARE.7.gz

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
       first 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 first 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)