public Set < Symbol > getCommonGroupingColumns ( ) { Set < Symbol > intersection = new HashSet <> ( groupingSets . get ( 0 ) ) ; for ( int i = 1 ; i < groupingSets . size ( ) ; i ++ ) { intersection . retainAll ( groupingSets . get ( i ) ) ; } return ImmutableSet . copyOf ( intersection ) ; }
returns the common grouping columns in terms of output symbols