bionic (7) PREPARE.7.gz

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
       first 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 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

       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 first 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

       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.

       To examine the query plan PostgreSQL is using for a prepared statement, use EXPLAIN(7), 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 PostgreSQL for that purpose, see
       the ANALYZE(7) documentation.

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

       DEALLOCATE(7), EXECUTE(7)