PREPARE(7) SQL Commands PREPARE(7)
PREPARE - prepare a statement for execution
PREPARE name [ ( datatype [, ...] ) ] AS statement
PREPARE creates a prepared statement. A prepared statement is a server-side object that can be used to optimize performance. When the PRE-
PARE 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
[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. The prepared statement can be manually cleaned up
using the DEALLOCATE [deallocate(7)] command.
Prepared statements have the largest performance advantage when a single session is being used to execute a large number of similar state-
ments. 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 re-
write 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.
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.
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 might be suboptimal. To exam-
ine the query plan PostgreSQL has chosen for a prepared statement, use EXPLAIN [explain(7)].
For more information on query planning and the statistics collected by PostgreSQL for that purpose, see the ANALYZE [analyze(7)] documenta-
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.
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.
DEALLOCATE [deallocate(7)], EXECUTE [execute(7)]
SQL - Language Statements 2010-05-14 PREPARE(7)