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)