(2018/07/05 20:15), Etsuro Fujita wrote:
> (2018/07/04 19:11), Ashutosh Bapat wrote:
>> On Wed, Jul 4, 2018 at 3:36 PM, Etsuro Fujita
>> <fujita.etsuro@lab.ntt.co.jp> wrote:
>>>
>>> I don't produce a test case where the plan is an Append with Gather
>>> subplans, but I'm not sure that it's a good thing to allow us to
>>> consider
>>> such a plan because in that plan, each Gather would try to grab its
>>> own pool
>>> of workers. Am I missing something?
>>
>> If the overall join can not use parallelism, having individual child
>> joins use parallel query might turn out to be efficient even if done
>> one child at a time. Parallel append drastically reduces the cases
>> where something like could be useful, but I don't think we can
>> theoretically eliminate the need for such a plan.
>
> In the case where scanjoin_target_parallel_safe=false, we actually will
> consider such parallel paths using the existing partial paths for the
> parent appendrel in the code path shown in a previous email (note: we
> would already have done generate_partitionwise_join_paths or
> set_append_rel_pathlist for the parent appendrel in query_planner.) For
> such a parallel path, we might need to do a projection in the Gather
> node for generating the SRF-free scan/join target, but I think that
> would be still efficient. So, I'm not sure that we really need to create
> child Gathers and generate an Append with those Gathers, as you mentioned.
>
> Another thing I noticed is: actually, we don't produce an Append with
> child Gathers in apply_scanjoin_target_to_paths, which I thought we
> would do that in the case of scanjoin_target_parallel_safe=false, but I
> noticed I was wrong. Sorry for that. The reason is because in that case,
> even if we create new partial Append paths with child Gathers, we don't
> run generate_gatehr_paths for the newly created partial paths at the end
> of that function shown below, since the parent's consider_parallel flag
> is set to false in that case:
>
> /*
> * Consider generating Gather or Gather Merge paths. We must only do this
> * if the relation is parallel safe, and we don't do it for child rels to
> * avoid creating multiple Gather nodes within the same plan. We must do
> * this after all paths have been generated and before set_cheapest, since
> * one of the generated paths may turn out to be the cheapest one.
> */
> if (rel->consider_parallel && !IS_OTHER_REL(rel))
> generate_gather_paths(root, rel, false);
One thing to add is: ISTM we need some cleanup for
apply_scanjoin_target_to_paths.
Best regards,
Etsuro Fujita
