presto-main/src/main/java/io/prestosql/sql/planner/plan/SetOperationNode.java/SetOperationNode/sourceOutputLayout
class SetOperationNode:    public List<Symbol> sourceOutputLayout(int sourceIndex)
    {
        // Make sure the sourceOutputLayout symbols are listed in the same order as the corresponding output symbols
        return getOutputSymbols().stream()
                .map(symbol -> outputToInputs.get(symbol).get(sourceIndex))
                .collect(toImmutableList());
    }