PREPARE(7)

1PREPARE(7)              PostgreSQL 9.2.24 Documentation             PREPARE(7)
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NAME

6       PREPARE - prepare a statement for execution
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SYNOPSIS

9       PREPARE name [ ( data_type [, ...] ) ] AS statement
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DESCRIPTION

12       PREPARE creates a prepared statement. A prepared statement is a
13       server-side object that can be used to optimize performance. When the
14       PREPARE statement is executed, the specified statement is parsed,
15       analyzed, and rewritten. When an EXECUTE command is subsequently
16       issued, the prepared statement is planned and executed. This division
17       of labor avoids repetitive parse analysis work, while allowing the
18       execution plan to depend on the specific parameter values supplied.
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20       Prepared statements can take parameters: values that are substituted
21       into the statement when it is executed. When creating the prepared
22       statement, refer to parameters by position, using $1, $2, etc. A
23       corresponding list of parameter data types can optionally be specified.
24       When a parameter's data type is not specified or is declared as
25       unknown, the type is inferred from the context in which the parameter
26       is used (if possible). When executing the statement, specify the actual
27       values for these parameters in the EXECUTE statement. Refer to
28       EXECUTE(7) for more information about that.
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30       Prepared statements only last for the duration of the current database
31       session. When the session ends, the prepared statement is forgotten, so
32       it must be recreated before being used again. This also means that a
33       single prepared statement cannot be used by multiple simultaneous
34       database clients; however, each client can create their own prepared
35       statement to use. Prepared statements can be manually cleaned up using
36       the DEALLOCATE(7) command.
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38       Prepared statements have the largest performance advantage when a
39       single session is being used to execute a large number of similar
40       statements. The performance difference will be particularly significant
41       if the statements are complex to plan or rewrite, for example, if the
42       query involves a join of many tables or requires the application of
43       several rules. If the statement is relatively simple to plan and
44       rewrite but relatively expensive to execute, the performance advantage
45       of prepared statements will be less noticeable.
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PARAMETERS

48       name
49           An arbitrary name given to this particular prepared statement. It
50           must be unique within a single session and is subsequently used to
51           execute or deallocate a previously prepared statement.
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53       data_type
54           The data type of a parameter to the prepared statement. If the data
55           type of a particular parameter is unspecified or is specified as
56           unknown, it will be inferred from the context in which the
57           parameter is used. To refer to the parameters in the prepared
58           statement itself, use $1, $2, etc.
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60       statement
61           Any SELECT, INSERT, UPDATE, DELETE, or VALUES statement.
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NOTES

64       If a prepared statement is executed enough times, the server may
65       eventually decide to save and re-use a generic plan rather than
66       re-planning each time. This will occur immediately if the prepared
67       statement has no parameters; otherwise it occurs only if the generic
68       plan appears to be not much more expensive than a plan that depends on
69       specific parameter values. Typically, a generic plan will be selected
70       only if the query's performance is estimated to be fairly insensitive
71       to the specific parameter values supplied.
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73       To examine the query plan PostgreSQL is using for a prepared statement,
74       use EXPLAIN(7). If a generic plan is in use, it will contain parameter
75       symbols $n, while a custom plan will have the current actual parameter
76       values substituted into it.
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78       For more information on query planning and the statistics collected by
79       PostgreSQL for that purpose, see the ANALYZE(7) documentation.
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81       You can see all prepared statements available in the session by
82       querying the pg_prepared_statements system view.
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EXAMPLES

85       Create a prepared statement for an INSERT statement, and then execute
86       it:
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88           PREPARE fooplan (int, text, bool, numeric) AS
89               INSERT INTO foo VALUES($1, $2, $3, $4);
90           EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);
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92       Create a prepared statement for a SELECT statement, and then execute
93       it:
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95           PREPARE usrrptplan (int) AS
96               SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid
97               AND l.date = $2;
98           EXECUTE usrrptplan(1, current_date);
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100       Note that the data type of the second parameter is not specified, so it
101       is inferred from the context in which $2 is used.
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COMPATIBILITY

104       The SQL standard includes a PREPARE statement, but it is only for use
105       in embedded SQL. This version of the PREPARE statement also uses a
106       somewhat different syntax.
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