presto-main/src/main/java/io/prestosql/type/setdigest/BuildSetDigestAggregation.java/BuildSetDigestAggregation/combine
class BuildSetDigestAggregation:    @CombineFunction
    public static void combine(SetDigestState state, SetDigestState otherState)
    {
        if (state.getDigest() == null) {
            SetDigest copy = new SetDigest();
            copy.mergeWith(otherState.getDigest());
            state.setDigest(copy);
        }
        else {
            state.getDigest().mergeWith(otherState.getDigest());
        }
    }