PREPARE - prepare a statement for execution
data_type[, ...] ) ] AS
creates a prepared statement. A prepared
statement is a server-side object that can be used to optimize
performance. When the
executed, the specified statement is parsed, analyzed, and rewritten.
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
, 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
, 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
statement. Refer to
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.
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.
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 referenced. To refer to the parameters in the prepared statement itself, use
Prepared statements can use generic plans rather than re-planning with
each set of supplied
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.
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
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
for a prepared statement, use
If a generic plan is in use, it will contain parameter symbols
, 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 documentation.
Although the main point of a prepared statement is to avoid repeated parse
analysis and planning of the statement,
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
from one use to the next, the statement will be re-parsed using the new
. (This latter behavior is new as of
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
, 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
You can see all prepared statements available in the session by querying the
Create a prepared statement for an
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
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
The SQL standard includes a
but it is only for use in embedded SQL. This version of the
statement also uses a somewhat different