Provided by: postgresql-client-8.0_8.0.7-2build1_i386
PREPARE - prepare a statement for execution
PREPARE plan_name [ (datatype [, ...] ) ] AS statement
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(7)] for more information about
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 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.
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.
The data type of a parameter to the prepared statement. To refer
to the parameters in the prepared statement itself, use $1, $2,
Any SELECT, INSERT, UPDATE, or DELETE statement.
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 may 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)]
Create a prepared query 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 query for a SELECT statement, and then execute it:
PREPARE usrrptplan (int, date) 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);
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.
DEALLOCATE [deallocate(7)], EXECUTE [execute(l)]