PREPARE — prepare a statement for execution
PREPAREname
[ (data_type
[, ...] ) ] ASstatement
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.
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 used. To refer to the
parameters in the prepared statement itself, use
$1
, $2
, etc.
statement
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
$
, while a custom plan will have the
current actual parameter values substituted into it.
n
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 subsequent
uses.
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 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.