SPI_freetuptable frees a row set created by a
   prior SPI command execution function, such as
   SPI_execute.  Therefore, this function is usually called
   with the global variable SPI_tupletable as
   argument.
  
   This function is useful if a SPI procedure needs to execute
   multiple commands and does not want to keep the results of earlier
   commands around until it ends.  Note that any unfreed row sets will
   be freed anyway at SPI_finish.