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.