prepare(7) - phpMan

PREPARE(7) PostgreSQL 9.2.24 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 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.

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 used. To refer to the parameters in the
prepared statement itself, use $1, $2, etc.

statement
Any SELECT, INSERT, UPDATE, DELETE, or VALUES statement.

NOTES
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.

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);

Note that 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.