SPI_saveplan saves a passed plan
(prepared by SPI_prepare) in memory
that will not be freed by SPI_finish nor by the transaction manager, and
returns a pointer to the saved plan. This gives you the ability
to reuse prepared plans in the subsequent invocations of your
procedure in the current session.
Pointer to the saved plan; NULL if unsuccessful. On error, SPI_result is set thus:
if plan is NULL or invalid
if called from an unconnected procedure
The passed-in plan is not freed, so you might wish to do
SPI_freeplan on it to avoid leaking
memory until SPI_finish.
If one of the objects (a table, function, etc.) referenced by
the prepared plan is dropped or redefined, then future executions
of SPI_execute_plan may fail or
return different results than the plan initially indicates.