presto-main/src/main/java/io/prestosql/sql/planner/plan/UnionNode.java/UnionNode/UnionNode
class UnionNode:    @JsonCreator
    public UnionNode(
            @JsonProperty("id") PlanNodeId id,
            @JsonProperty("sources") List<PlanNode> sources,
            @JsonProperty("outputToInputs") ListMultimap<Symbol, Symbol> outputToInputs,
            @JsonProperty("outputs") List<Symbol> outputs)
    {
        super(id, sources, outputToInputs, outputs);
    }