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.