Re: Proposal : Parallel Merge Join

From: Peter Geoghegan <pg(at)heroku(dot)com>
To: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Proposal : Parallel Merge Join
Date: 2016-12-11 02:44:02
Message-ID: CAM3SWZTF2x1Wa+d95O2s7pKd2y7r+yidMNAB8B__tXA2E62xQw@mail.gmail.com
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On Sat, Dec 10, 2016 at 4:59 AM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> 3. 20_patch.out : Explain analyze output with patch (median of 3 runs)

I noticed that the partially parallelized Merge Join EXPLAIN ANALYZE
that you attach has a big misestimation:

-> Merge Join (cost=3405052.45..3676948.66 rows=320 width=32)
(actual time=21165.849..37814.551 rows=1357812 loops=4)

Is this the best test case to show off the patch? This node is the
immediate outer child of a Nested Loop Semi Join, and so I'm concerned
that we measuring the wrong thing.

--
Peter Geoghegan

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