Prepare a statement for execution.
PREPARE <name> [ (<datatype> [, ...] ) ] AS <statement>
PREPARE creates a prepared statement, possibly with unbound parameters. A prepared statement is a server-side object that can be used to optimize performance. A prepared statement may be subsequently executed with a binding for its parameters. HAWQ may choose to replan the query for different executions of the same prepared statement.
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
$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
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.
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. HAWQ 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 HAWQ has chosen for a prepared statement, use
For more information on query planning and the statistics collected by HAWQ for that purpose, see the
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. Note that the data type of the second parameter is not specified, so it is inferred from the context in which
$2 is used:
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);
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.