Re: Parallel Hash take II

From: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
To: Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>
Cc: Prabhat Sahu <prabhat(dot)sahu(at)enterprisedb(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Rafia Sabih <rafia(dot)sabih(at)enterprisedb(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Oleg Golovanov <rentech(at)mail(dot)ru>
Subject: Re: Parallel Hash take II
Date: 2017-11-03 01:05:24
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On Mon, Oct 30, 2017 at 1:49 PM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> A couple of stupid things outstanding:
> 1. EXPLAIN ANALYZE for Parallel Hash "actual" shows the complete row
> count, which is interesting to know (or not? maybe I should show it
> somewhere else?), but the costing shows the partial row estimate used
> for costing purposes.


> 2. The BufFileSet's temporary directory gets created even if you
> don't need it for batches. Duh.


I also refactored shared temporary files a bit while looking into
this. The shared file ownership mechanism is now promoted to its own
translation unit sharedfileset.c and it now works with fd.c files.
buffile.c can still make use of it. That seems like a better division
of labour.

> 3. I don't have a good query rescan regression query yet. I wish I
> could write my own query plans to test the executor.

I found a query that rescans a parallel-aware hash join and added a
couple of variants to the regression tests.

I also decluttered the EXPLAIN ANALYZE output for enable_parallel_hash
= off a bit.

Thomas Munro

Attachment Content-Type Size
parallel-hash-v24.patchset.tgz application/x-gzip 65.2 KB

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