presto-main/src/main/java/io/prestosql/operator/aggregation/MapUnionAggregation.java/MapUnionAggregation/MapUnionAggregation
class MapUnionAggregation:    public MapUnionAggregation()
    {
        super(
                new FunctionMetadata(
                        new Signature(
                                NAME,
                                ImmutableList.of(comparableTypeParameter("K"), typeVariable("V")),
                                ImmutableList.of(),
                                mapType(new TypeSignature("K"), new TypeSignature("V")),
                                ImmutableList.of(mapType(new TypeSignature("K"), new TypeSignature("V"))),
                                false),
                        true,
                        ImmutableList.of(new FunctionArgumentDefinition(false)),
                        false,
                        true,
                        "Aggregate all the maps into a single map",
                        AGGREGATE),
                true,
                false);
    }