SPI_returntuple makes a copy of a row in
   the upper executor context, returning it in the form of a row type Datum.
   The returned pointer need only be converted to Datum via PointerGetDatum
   before returning.
  
   Note that this should be used for functions that are declared to return
   composite types.  It is not used for triggers; use
   SPI_copytuple for returning a modified row in a trigger.