PREPARE(7) PostgreSQL 15.7 Documentation PREPARE(7)
NAME
PREPARE - prepare a statement for execution
SYNOPSIS
PREPARE name [ ( data_type [, ...] ) ] 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,
analyzed, and rewritten. When an EXECUTE command is subsequently
issued, the prepared statement is planned and executed. This division
of labor avoids repetitive parse analysis work, while allowing the
execution plan to depend on the specific parameter values supplied.
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 first referenced (if possible). When executing the statement,
specify the actual values for these parameters in the EXECUTE
statement. Refer to 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. Prepared statements can be manually cleaned up using
the DEALLOCATE command.
Prepared statements potentially 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, e.g., 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.
data_type
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 first referenced. To refer to the parameters in the
prepared statement itself, use $1, $2, etc.
statement
Any SELECT, INSERT, UPDATE, DELETE, MERGE, or VALUES statement.
NOTES
A prepared statement can be executed with either a generic plan or a
custom plan. A generic plan is the same across all executions, while a
custom plan is generated for a specific execution using the parameter
values given in that call. Use of a generic plan avoids planning
overhead, but in some situations a custom plan will be much more
efficient to execute because the planner can make use of knowledge of
the parameter values. (Of course, if the prepared statement has no
parameters, then this is moot and a generic plan is always used.)
By default (that is, when plan_cache_mode is set to auto), the server
will automatically choose whether to use a generic or custom plan for a
prepared statement that has parameters. The current rule for this is
that the first five executions are done with custom plans and the
average estimated cost of those plans is calculated. Then a generic
plan is created and its estimated cost is compared to the average
custom-plan cost. Subsequent executions use the generic plan if its
cost is not so much higher than the average custom-plan cost as to make
repeated replanning seem preferable.
This heuristic can be overridden, forcing the server to use either
generic or custom plans, by setting plan_cache_mode to
force_generic_plan or force_custom_plan respectively. This setting is
primarily useful if the generic plan's cost estimate is badly off for
some reason, allowing it to be chosen even though its actual cost is
much more than that of a custom plan.
To examine the query plan PostgreSQL is using for a prepared statement,
use EXPLAIN, for example
EXPLAIN EXECUTE name(parameter_values);
If a generic plan is in use, it will contain parameter symbols $n,
while a custom plan will have the supplied parameter values substituted
into it.
For more information on query planning and the statistics collected by
PostgreSQL for that purpose, see the ANALYZE(7) documentation.
Although the main point of a prepared statement is to avoid repeated
parse analysis and planning of the statement, PostgreSQL will force
re-analysis and re-planning of the statement before using it whenever
database objects used in the statement have undergone definitional
(DDL) changes or their planner statistics have been updated since the
previous use of the prepared statement. Also, if the value of
search_path changes from one use to the next, the statement will be
re-parsed using the new search_path. (This latter behavior is new as of
PostgreSQL 9.3.) These rules make use of a prepared statement
semantically almost equivalent to re-submitting the same query text
over and over, but with a performance benefit if no object definitions
are changed, especially if the best plan remains the same across uses.
An example of a case where the semantic equivalence is not perfect is
that if the statement refers to a table by an unqualified name, and
then a new table of the same name is created in a schema appearing
earlier in the search_path, no automatic re-parse will occur since no
object used in the statement changed. However, if some other change
forces a re-parse, the new table will be referenced in subsequent uses.
You can see all prepared statements available in the 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);
In this example, 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(7), EXECUTE(7)
PostgreSQL 15.7 2024 PREPARE(7)
Generated by dwww version 1.15 on Sat Jun 29 02:30:27 CEST 2024.