presto-main/src/main/java/io/prestosql/sql/planner/plan/SetOperationNode.java/SetOperationNode/sourceSymbolMap
class SetOperationNode:    public Map<Symbol, SymbolReference> sourceSymbolMap(int sourceIndex)
    {
        ImmutableMap.Builder<Symbol, SymbolReference> builder = ImmutableMap.builder();
        for (Map.Entry<Symbol, Collection<Symbol>> entry : outputToInputs.asMap().entrySet()) {
            builder.put(entry.getKey(), Iterables.get(entry.getValue(), sourceIndex).toSymbolReference());
        }

        return builder.build();
    }