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)