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PREPARE(5)


NAME

  PREPARE - prepare a statement for execution


SYNOPSIS

  PREPARE plan_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. To include parameters in a prepared
  statement, supply a list of data types in the PREPARE statement, and, in
  the statement to be prepared itself, refer to the parameters by position
  using $1, $2, etc. When executing the statement, specify the actual values
  for these parameters in the EXECUTE statement. Refer to EXECUTE
  [execute(5)] for more information about that.

  Prepared statements are only stored in and for the duration of the current
  database session. When the session ends, the prepared statement is
  forgotten, and 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 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

  plan_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. To refer to
       the parameters in the prepared statement itself, use $1, $2, etc.

  statement
       Any SELECT, INSERT, UPDATE, or DELETE statement.


NOTES

  In some situations, the query plan produced by for a prepared statement may
  be inferior to the plan produced if the statement were 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 may be suboptimal. To
  examine the query plan PostgreSQL has chosen for a prepared statement, use
  EXPLAIN EXECUTE.

  For more information on query planning and the statistics collected by
  PostgreSQL for that purpose, see the ANALYZE [analyze(5)] documentation.


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.

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