1SPI_PREPARE(3) PostgreSQL 12.2 Documentation SPI_PREPARE(3)
2
6 SPI_prepare - prepare a statement, without executing it yet
7
9 SPIPlanPtr SPI_prepare(const char * command, int nargs, Oid * argtypes)
10
12 SPI_prepare creates and returns a prepared statement for the specified
13 command, but doesn't execute the command. The prepared statement can
14 later be executed repeatedly using SPI_execute_plan.
15 When the same or a similar command is to be executed repeatedly, it is
17 generally advantageous to perform parse analysis only once, and might
18 furthermore be advantageous to re-use an execution plan for the
19 command. SPI_prepare converts a command string into a prepared
20 statement that encapsulates the results of parse analysis. The prepared
21 statement also provides a place for caching an execution plan if it is
22 found that generating a custom plan for each execution is not helpful.
23 A prepared command can be generalized by writing parameters ($1, $2,
25 etc.) in place of what would be constants in a normal command. The
26 actual values of the parameters are then specified when
27 SPI_execute_plan is called. This allows the prepared command to be used
28 over a wider range of situations than would be possible without
29 parameters.
30 The statement returned by SPI_prepare can be used only in the current
32 invocation of the C function, since SPI_finish frees memory allocated
33 for such a statement. But the statement can be saved for longer using
34 the functions SPI_keepplan or SPI_saveplan.
35
37 const char * command
38 command string
39 int nargs
41 number of input parameters ($1, $2, etc.)
42 Oid * argtypes
44 pointer to an array containing the OIDs of the data types of the
45 parameters
46
48 SPI_prepare returns a non-null pointer to an SPIPlan, which is an
49 opaque struct representing a prepared statement. On error, NULL will be
50 returned, and SPI_result will be set to one of the same error codes
51 used by SPI_execute, except that it is set to SPI_ERROR_ARGUMENT if
52 command is NULL, or if nargs is less than 0, or if nargs is greater
53 than 0 and argtypes is NULL.
54
56 If no parameters are defined, a generic plan will be created at the
57 first use of SPI_execute_plan, and used for all subsequent executions
58 as well. If there are parameters, the first few uses of
59 SPI_execute_plan will generate custom plans that are specific to the
60 supplied parameter values. After enough uses of the same prepared
61 statement, SPI_execute_plan will build a generic plan, and if that is
62 not too much more expensive than the custom plans, it will start using
63 the generic plan instead of re-planning each time. If this default
64 behavior is unsuitable, you can alter it by passing the
65 CURSOR_OPT_GENERIC_PLAN or CURSOR_OPT_CUSTOM_PLAN flag to
66 SPI_prepare_cursor, to force use of generic or custom plans
67 respectively.
68 Although the main point of a prepared statement is to avoid repeated
70 parse analysis and planning of the statement, PostgreSQL will force
71 re-analysis and re-planning of the statement before using it whenever
72 database objects used in the statement have undergone definitional
73 (DDL) changes since the previous use of the prepared statement. Also,
74 if the value of search_path changes from one use to the next, the
75 statement will be re-parsed using the new search_path. (This latter
76 behavior is new as of PostgreSQL 9.3.) See PREPARE(7) for more
77 information about the behavior of prepared statements.
78 This function should only be called from a connected C function.
80 SPIPlanPtr is declared as a pointer to an opaque struct type in spi.h.
82 It is unwise to try to access its contents directly, as that makes your
83 code much more likely to break in future revisions of PostgreSQL.
84 The name SPIPlanPtr is somewhat historical, since the data structure no
86 longer necessarily contains an execution plan.
87PostgreSQL 12.2 2020 SPI_PREPARE(3)