Re: [HACKERS] Partition-wise aggregation/grouping

From: Rafia Sabih <rafia(dot)sabih(at)enterprisedb(dot)com>
To: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, 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: 2018-02-14 06:47:37
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On Tue, Feb 13, 2018 at 6:21 PM, Jeevan Chalke <
jeevan(dot)chalke(at)enterprisedb(dot)com> wrote:

> I see that partition-wise aggregate plan too uses parallel index, am I
> missing something?
You're right, I missed that, oops.

>> Q18 takes some 390 secs with patch and some 147 secs without it.
> This looks strange. This patch set does not touch parallel or seq scan as
> such. I am not sure why this is happening. All these three queries explain
> plan shows much higher execution time for parallel/seq scan.
> Yeah strange it is.

> However, do you see similar behaviour with patches applied,
> "enable_partition_wise_agg = on" and "enable_partition_wise_agg = off" ?

I tried that for query 18, with patch and enable_partition_wise_agg = off,
query completes in some 270 secs. You may find the explain analyse output
for it in the attached file. I noticed that on head the query plan had
parallel hash join however with patch and no partition-wise agg it is using
nested loop joins. This might be the issue.

> Also, does rest of the queries perform better with partition-wise
> aggregates?
As far as this setting goes, there wasn't any other query using
partition-wise-agg, so, no.

BTW, just an FYI, this experiment is on scale factor 20.

Rafia Sabih

Attachment Content-Type Size
18_pwa_off.out application/octet-stream 10.0 KB

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