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.