prepare(7)

1PREPARE(7)                       SQL Commands                       PREPARE(7)
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NAME

6       PREPARE - prepare a statement for execution
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SYNOPSIS

10       PREPARE name [ ( datatype [, ...] ) ] AS statement
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DESCRIPTION

14       PREPARE creates a prepared statement. A prepared statement is a server-
15       side object that can be used to optimize performance. When the  PREPARE
16       statement  is  executed,  the specified statement is parsed, rewritten,
17       and planned. When an EXECUTE command is subsequently issued,  the  pre‐
18       pared  statement  need  only be executed. Thus, the parsing, rewriting,
19       and planning stages are only performed once, instead of every time  the
20       statement is executed.
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22       Prepared  statements  can  take parameters: values that are substituted
23       into the statement when it is  executed.  When  creating  the  prepared
24       statement, refer to parameters by position, using $1, $2, etc. A corre‐
25       sponding list of parameter data types can optionally be specified. When
26       a parameter's data type is not specified or is declared as unknown, the
27       type is inferred from the context in which the parameter  is  used  (if
28       possible).  When executing the statement, specify the actual values for
29       these parameters in the  EXECUTE  statement.  Refer  to  EXECUTE  [exe‐
30       cute(7)] for more information about that.
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32       Prepared  statements only last for the duration of the current database
33       session. When the session ends, the prepared statement is forgotten, so
34       it  must  be  recreated before being used again. This also means that a
35       single prepared statement cannot be used by multiple simultaneous data‐
36       base clients; however, each client can create their own prepared state‐
37       ment to use. The prepared statement can be manually  cleaned  up  using
38       the DEALLOCATE [deallocate(7)] command.
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40       Prepared  statements have the largest performance advantage when a sin‐
41       gle session is being used to execute a large number of  similar  state‐
42       ments.  The  performance difference will be particularly significant if
43       the statements are complex to plan or  rewrite,  for  example,  if  the
44       query  involves  a  join  of many tables or requires the application of
45       several rules. If the statement is relatively simple to  plan  and  re‐
46       write but relatively expensive to execute, the performance advantage of
47       prepared statements will be less noticeable.
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PARAMETERS

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

65       In some situations, the query plan produced for  a  prepared  statement
66       will  be  inferior to the query plan that would have been chosen if the
67       statement had been submitted and executed  normally.  This  is  because
68       when the statement is planned and the planner attempts to determine the
69       optimal query plan, the actual values of any  parameters  specified  in
70       the  statement  are  unavailable. PostgreSQL collects statistics on the
71       distribution of data in the table, and can use  constant  values  in  a
72       statement  to  make  guesses  about  the likely result of executing the
73       statement. Since this data is unavailable when planning prepared state‐
74       ments  with parameters, the chosen plan might be suboptimal. To examine
75       the query plan PostgreSQL has chosen  for  a  prepared  statement,  use
76       EXPLAIN [explain(7)].
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78       For  more information on query planning and the statistics collected by
79       PostgreSQL for that purpose, see the  ANALYZE  [analyze(7)]  documenta‐
80       tion.
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82       You  can see all available prepared statements of a session by querying
83       the pg_prepared_statements system view.
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EXAMPLES

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

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