SPI_freeplan — free a previously saved prepared statement
int SPI_freeplan(SPIPlanPtr plan)
   SPI_freeplan releases a prepared statement
   previously returned by SPI_prepare or saved by
   SPI_keepplan or SPI_saveplan.
  
SPIPlanPtr planpointer to statement to free
   0 on success;
   SPI_ERROR_ARGUMENT if plan
   is NULL or invalid