Support BYTES type for group-by expression

[Jackie-Jiang]

Description

Support BYTES type for group-by expression

Changes:

• Add ValueToIdMap (on-the-fly dictionary) for BYTES type
• Re-order the operation to save the per-value switch case in NoDictionaryMultiColumnGroupKeyGenerator. generateKeysForBlock()
• Add type specific group key iterator in NoDictionaryMultiColumnGroupKeyGenerator for performance improvement
• Enhance NoDictionaryGroupKeyGeneratorTest to test BYTES type

[siddharthteotia]

This may sound like an over-optimization but adding type specific GroupKeyGenerator has the potential to add overhead for runtime dispatch which Java doesn't handle well.

What is the advantage of having a type specific next method like the following for Double

@Override
    public GroupKey next() {
      Double2IntMap.Entry entry = _iterator.next();
      _groupKey._groupId = entry.getIntValue();
      _groupKey._stringKey = Double.toString(entry.getDoubleKey());
      return _groupKey;
    }

v/s the existing generic method

@Override
    public GroupKey next() {
      Map.Entry<Object, Integer> entry = _iterator.next();
      _groupKey._groupId = entry.getValue();
      _groupKey._stringKey = entry.getKey().toString();
      return _groupKey;
    }

Both are doing toString()

[Jackie-Jiang]

The main difference is from the entry-set and iterator implementation of the fastutil maps. For an example, Int2IntMap.entrySet() is marked deprecated and the class suggest using the type-specific method instead. Also, we are able to use the fastIterator() provided by the FastEntrySet, and also the unboxed values to reduce the garbage, and the unnecessary boxing/unboxing.