PREPARE(7)                                                         SQL Commands                                                         PREPARE(7)

NAME

       PREPARE - prepare a statement for execution

SYNOPSIS

       PREPARE name [ ( datatype [, ...] ) ] 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 PRE-
       PARE 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
       [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 [deallocate(7)] command.

       Prepared  statements have the largest performance advantage when a single session is being used to execute a large number of similar state-
       ments. 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 re-
       write 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.

       datatype
              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 exam-
       ine the query plan PostgreSQL has chosen for a prepared statement, use EXPLAIN [explain(7)].

       For more information on query planning and the statistics collected by PostgreSQL for that purpose, see the ANALYZE [analyze(7)] documenta-
       tion.

       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 [deallocate(7)], EXECUTE [execute(7)]

SQL - Language Statements                                           2010-05-14                                                          PREPARE(7)