presto-main/src/main/java/io/prestosql/sql/planner/plan/SetOperationNode.java/SetOperationNode/outputSymbolMap
class SetOperationNode:    public Multimap<Symbol, SymbolReference> outputSymbolMap(int sourceIndex)
    {
        return Multimaps.transformValues(FluentIterable.from(getOutputSymbols())
                .toMap(outputToSourceSymbolFunction(sourceIndex))
                .asMultimap()
                .inverse(), Symbol::toSymbolReference);
    }