PREPARE - prepare a statement for execution
PREPARE name [ ( data_type [, ...] ) ] 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,
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 used (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(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.
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 used. To refer to the parameters in the prepared
statement itself, use $1, $2, etc.
Any SELECT, INSERT, UPDATE, DELETE, or VALUES statement.
If a prepared statement is executed enough times, the server may
eventually decide to save and re-use a generic plan rather than
re-planning each time. This will occur immediately if the prepared
statement has no parameters; otherwise it occurs only if the generic
plan appears to be not much more expensive than a plan that depends on
specific parameter values. Typically, a generic plan will be selected
only if the query's performance is estimated to be fairly insensitive
to the specific parameter values supplied.
To examine the query plan PostgreSQL is using for a prepared statement,
use EXPLAIN(7). If a generic plan is in use, it will contain parameter
symbols $n, while a custom plan will have the current actual 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 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
You can see all prepared statements available in the session by
querying the pg_prepared_statements system view.
Create a prepared statement for an INSERT statement, and then execute
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
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.
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.