Re: [HACKERS] Partition-wise aggregation/grouping

From: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Rajkumar Raghuwanshi <rajkumar(dot)raghuwanshi(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Partition-wise aggregation/grouping
Date: 2018-03-07 14:37:34
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On Wed, Mar 7, 2018 at 1:45 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Tue, Mar 6, 2018 at 5:31 AM, Jeevan Chalke
> <jeevan(dot)chalke(at)enterprisedb(dot)com> wrote:
> > This is in-lined with enable_hashagg GUC. Do you think
> > enable_partitionwise_aggregate seems better? But it will be not
> consistent
> > with other GUC names like enable_hashagg then.
> Well, if I had my way, enable_hashagg would be spelled
> enable_hash_aggregate, too, but I wasn't involved in the project that
> long ago. 100% consistency is hard to achieve here; the perfect
> parallel of enable_hashagg would be enable_partitionwiseagg, but then
> it would be inconsistent with enable_partitionwise_join unless we
> renamed it to enable_partitionwisejoin, which I would rather not do.
> I think the way the enable_blahfoo names were done was kinda
> shortsighted -- it works OK as long as blahfoo is pretty short, like
> mergejoin or hashagg or whatever, but if you have more or longer words
> then I think it's hard to see where the word boundaries are without
> any punctuation. And if you start abbreviating then you end up with
> things like enable_pwagg which are not very easy to understand. So I
> favor spelling everything out and punctuating it.

Understood and make sense.

> > So the code for doing partially aggregated partial paths and partially
> > aggregated non-partial path is same except partial paths goes into
> > partial_pathlist where as non-partial goes into pathlist of
> > partially_grouped_rel. Thus, calling add_paths_to_partial_grouping_rel()
> > when isPartialAgg = true seems correct. Also as we have decided, this
> > function is responsible to create all partially aggregated paths
> including
> > both partial and non-partial.
> >
> > Am I missing something?
> Hmm. I guess not. I think I didn't read this code well enough
> previously. Please find attached proposed incremental patches (0001
> and 0002) which hopefully make the code in this area a bit clearer.

Yep. Thanks for these patches.
I have merged these changes into my main (0007) patch now.

> >> + /*
> >> + * If there are any fully aggregated partial paths present,
> >> may be because
> >> + * of parallel Append over partitionwise aggregates, we must
> stick
> >> a
> >> + * Gather or Gather Merge path atop the cheapest partial path.
> >> + */
> >> + if (grouped_rel->partial_pathlist)
> >>
> >> This comment is copied from someplace where the code does what the
> >> comment says, but here it doesn't do any such thing.
> >
> > Well, these comments are not present anywhere else than this place. With
> > Parallel Append and Partitionwise aggregates, it is now possible to have
> > fully aggregated partial paths now. And thus we need to stick a Gather
> > and/or Gather Merge atop cheapest partial path. And I believe the code
> does
> > the same. Am I missing something?
> I misread the code. Sigh. I should have waited until today to send
> that email and taken time to study it more carefully. But I still
> don't think it's completely correct. It will not consider using a
> pre-sorted path; the only strategies it can consider are cheapest path
> + Gather and cheapest path + explicit Sort (even if the cheapest path
> is already correctly sorted!) + Gather Merge. It should really do
> something similar to what add_paths_to_partial_grouping_rel() already
> does: first call generate_gather_paths() and then, if the cheapest
> partial path is not already correctly sorted, also try an explicit
> Sort + Gather Merge. In fact, it looks like we can actually reuse
> that logic exactly. See attached 0003 incremental patch; this changes
> the outputs of one of your regression tests, but the new plan looks
> better.

This seems like a refactoring patch and thus added as separate patch (0005)
in patch-set.
Changes related to PWA patch are merged accordingly too.

Attached new patch-set with these changes merged and fixing review comments
from Ashutosh Bapat along with his GroupPathExtraData changes patch.

> Some other notes:
> There's a difference between performing partial aggregation in the
> same process and performing it in a different process. hasNonPartial
> tells us that we can't perform partial aggregation *at all*;
> hasNonSerial only tells us that partial and final aggregation must
> happen in the same process. This patch could possibly take advantage
> of partial aggregation even when hasNonSerial is set. Finalize
> Aggregate -> Append -> N copies of { Partial Aggregate -> Whatever }
> is OK with hasNonSerial = true as long as hasNonPartial = false. Now,
> the bad news is that for this to actually work we'd need to define new
> AGGSPLIT_FINAL = AGGSPLITOP_COMBINE, and I'm not sure how much
> complexity that adds. However, if we're not going to do that, I think
> we'd better at last add some comments about it suggesting that someone
> might want to do something about it in the future.

Am I not familier with these much. So will add a comment as you said.

> I think that, in general, it's a good idea to keep the number of times
> that create_grouping_paths() does something which is conditional on
> whether child_data is NULL to a minimum. I haven't looked at what
> Ashutosh tried to do there so I don't know whether it's good or bad,
> but I like the idea, if we can do it cleanly.
> It strikes me that we might want to consider refactoring things so
> that create_grouping_paths() takes the grouping_rel and
> partial_grouping_rel as input arguments. Right now, the
> initialization of the child grouping and partial-grouping rels is
> partly in try_partitionwise_aggregate(), which considers marking one
> of them (but never both?) as dummy rels and create_grouping_paths()
> which sets reloptkind, serverid, userid, etc. The logic of all of
> this is a little unclear to me. Presumably, if the input rel is
> dummy, then both the grouping_rel and the partial_grouping_rel are
> also dummy. Also, presumably we should set the reloptkind correctly
> as soon as we create the rel, not at some later stage.
> Or maybe what we should do is split create_grouping_paths() into two
> functions. Like this:
> if (child_data)
> {
> partial_grouping_target = child_data->partialRelTarget;
> partially_grouped_rel->reltarget = partial_grouping_target;
> agg_partial_costs = child_data->agg_partial_costs;
> agg_final_costs = child_data->agg_final_costs;
> }
> --- SPLIT IT HERE ---
> /* Apply partitionwise aggregation technique, if possible. */
> try_partitionwise_grouping(root, input_rel, grouped_rel,
> partially_grouped_rel, target,
> partial_grouping_target, agg_costs,
> agg_partial_costs, agg_final_costs, gd,
> can_sort, can_hash, havingQual,
> isPartialAgg);
> It seems to me that everything from that point to the end is doing the
> path generation and it's all pretty much the same for the parent and
> child cases. But everything before that is either stuff that doesn't
> apply to the child case at all (like the degenerate grouping case) or
> stuff that should be done once and passed down (like
> can_sort/can_hash). The only exception I see is some of the stuff
> that sets up the upper rel at the top of the function, but maybe that
> logic could be refactored into a separate function as well (like
> initialize_grouping_rel). Then, instead of try_partitionwise_join()
> actually calling create_grouping_paths(), it would call
> initialize_grouping_rel() and then the path-adding function that we
> split off from the bottom of the current create_grouping_paths(),
> basically skipping all that stuff in the middle that we don't really
> want to do in that case.

I will have a look over this proposal.

> --
> Robert Haas
> EnterpriseDB:
> The Enterprise PostgreSQL Company

Jeevan Chalke
Technical Architect, Product Development
EnterpriseDB Corporation
The Enterprise PostgreSQL Company

Attachment Content-Type Size
partition-wise-agg-v16.tar.gz application/gzip 32.9 KB

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