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.