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
may be advantageous to perform the planning only once.
SPI_prepare converts a command string into an
execution plan that can be executed repeatedly using
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
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.
There is a disadvantage to using parameters: since the planner does not know the values that will be supplied for the parameters, it may make worse planning choices than it would make for a normal command with all constants visible.