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.