PREPARE — prepare a statement for execution
data_type[, ...] ) ] AS
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.
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
$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 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.
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. 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
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
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
This heuristic can be overridden, forcing the server to use either
generic or custom plans, by setting
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
If a generic plan is in use, it will contain parameter symbols
$, 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 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 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
You can see all prepared statements available in the session by querying the
Create a prepared statement for an
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
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.
The SQL standard includes a
but it is only for use in embedded SQL. This version of the
PREPARE statement also uses a somewhat different