PartialMergeMulti does not support a Noop output

[adamretter]

It seems at the moment that MergeOperator::PartialMergeMulti must always produce a value.

However, consider that I have an operand list like so:

{
  operand[0]="Add(Item1)"
  operand[1]="Remove(Item1)"
}

My custom merge operator is able to realise that these two operations, actually reduce down to nothing i.e. no operation needs to be performed.

If I clear the output by calling new_value->clear(). The problem I see is that subsequent calls to MergeOperator::FullMergeV2 then have empty operands in the operand_list.
Whilst I could handle empty operands in my FullMergeV2, this seems like an unnecessary memory and processing overhead (especially if I have a large operand list, where all the operands are effectively empty).

I tried modifying db/merge_helper.cc (around line 341), so that if PartialMergeMulti results in an empty merge_result, then the result is ignored:

 if (merge_success) {
  // Merging of operands (associative merge) was successful.
  // Replace operands with the merge result
  merge_context_.Clear();
  if (!merge_result.empty()) {
    merge_context_.PushOperand(merge_result);
  }
  keys_.erase(keys_.begin(), keys_.end() - 1);
}

However I get assertion failures with regards to keys and values size. After further study it seems such a change is perhaps non-trivial to implement with the current design.

I do think that PartialMergeMulti being able to return nothing, is a valid state (as demonstrated by my custom MergeOperator above). Please advise as to the best way to move forward with this...