SPI_keepplan saves a passed statement (prepared by
SPI_prepare) so that it will not be freed
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.
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.