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 SPI_execp.
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_execp 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.
SPI_prepare returns non-null pointer to an execution plan. On error, NULL will be returned. In both cases, SPI_result will be set analogous to the value returned by SPI_exec, 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.