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.
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.