PREPARE(7) PostgreSQL 9.6.6 Documentation PREPARE(7)
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 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 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)
PostgreSQL 9.6.6 2017 PREPARE(7)
PostgreSQL 9.6.6 - Generated Sat Dec 2 12:31:27 CST 2017