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)]