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.