Re: [HACKERS] Partition-wise aggregation/grouping

From: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Jeevan Chalke <jeevan(dot)chalke(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: 2018-03-12 12:37:07
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On Fri, Mar 9, 2018 at 4:21 PM, Ashutosh Bapat
<ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> On Thu, Mar 8, 2018 at 7:31 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> This kind of goes along with the suggestion I made yesterday to
>> introduce a new function, which at the time I proposed calling
>> initialize_grouping_rel(), to set up new grouped or partially grouped
>> relations. By doing that it would be easier to ensure the
>> initialization is always done in a consistent way but only for the
>> relations we actually need. But maybe we should call it
>> fetch_grouping_rel() instead. The idea would be that instead of
>> calling fetch_upper_rel() we would call fetch_grouping_rel() when it
>> is a question of the grouped or partially grouped relation. It would
>> either return the existing relation or initialize a new one for us. I
>> think that would make it fairly easy to initialize only the ones we're
>> going to need.
> Hmm. I am working on refactoring the code to do something like this.

Here's patch doing the same. I have split create_grouping_paths() into
three functions 1. to handle degenerate grouping paths
(try_degenerate_grouping_paths()) 2. to create the grouping rels,
partial grouped rel and grouped rel (make_grouping_rels()), which also
sets some properties in GroupPathExtraData. 3. populate grouping rels
with paths (populate_grouping_rels_with_paths()). With those changes,
I have been able to get rid of partially grouped rels when they are
not necessary. But I haven't tried to get rid of grouped_rels when
they are not needed.

GroupPathExtraData now completely absorbs members from and replaces
OtherUpperPathExtraData. This means that we have to consider a way to
pass GroupPathExtraData to FDWs through GetForeignUpperPaths(). I
haven't tried it in this patch.

With this patch there's a failure in partition_aggregation where the
patch is creating paths with MergeAppend with GatherMerge underneath.
I think this is related to the call
add_paths_to_partial_grouping_rel() when try_parallel_aggregation is
true. But I didn't investigate it further.

With those two things remaining I am posting this patch, so that
Jeevan Chalke can merge this patch into his patches and also merge
some of his changes related to mine and Robert's changes. Let me know
if this refactoring looks good.

Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company

Attachment Content-Type Size
cgp_split.patch text/x-patch 37.5 KB

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