SPI_saveplan copies a passed statement (prepared by SPI_prepare) into memory that will not be freed by SPI_finish nor by the transaction manager, and returns a pointer to the copied statement. This gives you the ability to reuse prepared statements in the subsequent invocations of your procedure in the current session.
Pointer to the copied statement; or NULL if unsuccessful. On error, SPI_result is set thus:
if plan is NULL or invalid
if called from an unconnected procedure
The originally passed-in statement is not freed, so you might wish to do SPI_freeplan on it to avoid leaking memory until SPI_finish.
In most cases, SPI_keepplan is preferred to this function, since it accomplishes largely the same result without needing to physically copy the prepared statement's data structures.