SPI_prepare creates and returns a
prepared statement for the specified command, but doesn't execute
the command. The prepared statement can later be executed
repeatedly using SPI_execute_plan.
When the same or a similar command is to be executed repeatedly,
it is generally advantageous to perform parse analysis only once,
and might furthermore be advantageous to re-use an execution plan
for the command. SPI_prepare converts
a command string into a prepared statement that encapsulates the
results of parse analysis. The prepared statement also provides a
place for caching an execution plan if it is found that generating
a custom plan for each execution is not helpful.
A prepared command can be generalized by writing parameters
($1, $2, etc.) in
place of what would be constants in a normal command. The actual
values of the parameters are then specified when SPI_execute_plan is called. This allows the
prepared command to be used over a wider range of situations than
would be possible without parameters.
The statement returned by SPI_prepare can be used only in the current
invocation of the procedure, since SPI_finish frees memory allocated for such a
statement. But the statement can be saved for longer using the
functions SPI_keepplan or
SPI_saveplan.
command string
number of input parameters ($1, $2, etc.)
pointer to an array containing the OIDs of the data types of the parameters
SPI_prepare returns a non-null
pointer to an SPIPlan, which is an opaque
struct representing a prepared statement. On error, NULL will be returned, and SPI_result will be set to one of the same error
codes used by SPI_execute, except
that it is set to SPI_ERROR_ARGUMENT if
command is NULL,
or if nargs is less than 0, or if
nargs is greater than 0 and argtypes is NULL.
If no parameters are defined, a generic plan will be created at
the first use of SPI_execute_plan,
and used for all subsequent executions as well. If there are
parameters, the first few uses of SPI_execute_plan will generate custom plans that
are specific to the supplied parameter values. After enough uses of
the same prepared statement, SPI_execute_plan will build a generic plan, and
if that is not too much more expensive than the custom plans, it
will start using the generic plan instead of re-planning each time.
If this default behavior is unsuitable, you can alter it by passing
the CURSOR_OPT_GENERIC_PLAN or CURSOR_OPT_CUSTOM_PLAN flag to SPI_prepare_cursor, to force use of generic or
custom plans respectively.
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.) See PREPARE for more information about the behavior of prepared statements.
This function should only be called from a connected procedure.
SPIPlanPtr is declared as a pointer to an opaque struct type in spi.h. It is unwise to try to access its contents directly, as that makes your code much more likely to break in future revisions of PostgreSQL.
The name SPIPlanPtr is somewhat historical, since the data structure no longer necessarily contains an execution plan.