PREPARE — prepare a statement for execution
PREPAREname
[ (datatype
[, ...] ) ] 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, 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 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 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.
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.
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 may be suboptimal. To examine the query plan PostgreSQL has chosen for a prepared statement, use EXPLAIN.
For more information on query planning and the statistics collected by PostgreSQL for that purpose, see the ANALYZE documentation.
You can see all available prepared statements of a 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.