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 first referenced (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. Prepared statements can be manually cleaned up using the DEALLOCATE command.

Prepared statements potentially 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, e.g., 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

Notes

Prepared statements can use generic plans rather than re-planning with each set of supplied EXECUTE values. This occurs immediately for prepared statements with no parameters; otherwise it occurs only after five or more executions produce plans whose estimated cost average (including planning overhead) is more expensive than the generic plan cost estimate. Once a generic plan is chosen, it is used for the remaining lifetime of the prepared statement. Using EXECUTE values which are rare in columns with many duplicates can generate custom plans that are so much cheaper than the generic plan, even after adding planning overhead, that the generic plan might never be used.

A generic plan assumes that each value supplied to EXECUTE is one of the column's distinct values and that column values are uniformly distributed. For example, if statistics record three distinct column values, a generic plan assumes a column equality comparison will match 33% of processed rows. Column statistics also allow generic plans to accurately compute the selectivity of unique columns. Comparisons on non-uniformly-distributed columns and specification of non-existent values affects the average plan cost, and hence if and when a generic plan is chosen.

By default, generic plans for only 64 most recent prepared statements are kept in memory; older statements have to be parsed and analyzed again if they are called later. This limit is enforced by the plan_cache_lru_size configuration parameter, which enables you to control the amount of memory used by prepared statements. If you would like to keep plans in memory for all prepared statements as long as possible, set this parameter to 0.

To examine the query plan Postgres Pro is using for a prepared statement, use EXPLAIN, e.g., EXPLAIN EXECUTE. If a generic plan is in use, it will contain parameter symbols $n, while a custom plan will have the supplied parameter values substituted into it. The row estimates in the generic plan reflect the selectivity computed for the parameters.

For more information on query planning and the statistics collected by Postgres Pro for that purpose, see the ANALYZE documentation.

Although the main point of a prepared statement is to avoid repeated parse analysis and planning of the statement, Postgres Pro 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.

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.

See Also