SPI_prepare creates and returns
an execution plan for the specified command, but doesn't execute
the command. This function should only be called from a connected
procedure.
When the same or a similar command is to be executed
repeatedly, it might be advantageous to perform the planning only
once. SPI_prepare converts a
command string into an execution plan that can be executed
repeatedly using SPI_execute_plan.
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 plan returned by SPI_prepare
can be used only in the current invocation of the procedure,
since SPI_finish frees memory
allocated for a plan. But a plan can be saved for longer using
the function 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 execution plan. 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.
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.
There is a disadvantage to using parameters: since the planner does not know the values that will be supplied for the parameters, it might make worse planning choices than it would make for a normal command with all constants visible.