Provided by: postgresql-client-8.4_8.4.3-1_i386 bug

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  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
       [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
       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.

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