SPI_keepplan — save a prepared statement
int SPI_keepplan(SPIPlanPtr plan)   SPI_keepplan saves a passed statement (prepared by
   SPI_prepare) so that it will not be freed
   by SPI_finish nor by the transaction manager.
   This gives you the ability to reuse prepared statements in the subsequent
   invocations of your procedure in the current session.
  
SPIPlanPtr planthe prepared statement to be saved
   0 on success;
   SPI_ERROR_ARGUMENT if plan
   is NULL or invalid
  
   The passed-in statement is relocated to permanent storage by means
   of pointer adjustment (no data copying is required).  If you later
   wish to delete it, use SPI_freeplan on it.