Provided by: postgresql-client-8.4_8.4.11-1_amd64 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)]