Provided by: postgresql-client-9.1_9.1.3-2_amd64 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)