SPI_saveplan — save a prepared statement
SPIPlanPtr SPI_saveplan(SPIPlanPtr plan)   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.
  
SPIPlanPtr planthe prepared statement to be saved
   Pointer to the copied statement; or NULL if unsuccessful.
   On error, SPI_result is set thus:
   
SPI_ERROR_ARGUMENT       if plan is NULL or invalid
      
SPI_ERROR_UNCONNECTEDif 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.