MIME-Version: 1.0
References: <b3d80961-c2e5-38cc-6a32-61886cdf766d@gmail.com>
 <CAFiTN-v1yDvU=X+hwfJ+55=sbgDj=_kuvbduEG-F7=BjpWcnuw@mail.gmail.com>
 <efcb213c-c87c-41f0-85b7-7fa8079df807@enterprisedb.com> <CAApHDvqFtd-9DYH70sbjD7iB-Eq-xSip1LPr=nayfpPd1pkZVw@mail.gmail.com>
In-Reply-To: <CAApHDvqFtd-9DYH70sbjD7iB-Eq-xSip1LPr=nayfpPd1pkZVw@mail.gmail.com>
From: David Rowley <dgrowleyml@gmail.com>
Date: Tue, 9 Jul 2024 11:51:23 +1200
Message-ID: <CAApHDvoom6ABLgyNW3FtqtQxFB4Hz-6Y=zMy1EobPEOOMBDfUg@mail.gmail.com>
Subject: Re: Parallel Bitmap Heap Scan reports per-worker stats in EXPLAIN ANALYZE
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, David Geier <geidav.pg@gmail.com>, 
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Content-Type: multipart/mixed; boundary="000000000000e9a499061cc51749"
Archived-At: <https://www.postgresql.org/message-id/CAApHDvoom6ABLgyNW3FtqtQxFB4Hz-6Y%3DzMy1EobPEOOMBDfUg%40mail.gmail.com>
Precedence: bulk

--000000000000e9a499061cc51749
Content-Type: text/plain; charset="UTF-8"

On Mon, 8 Jul 2024 at 15:43, David Rowley <dgrowleyml@gmail.com> wrote:
>
> On Sun, 18 Feb 2024 at 11:31, Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
> > 2) Leader vs. worker counters
> >
> > It seems to me this does nothing to add the per-worker values from "Heap
> > Blocks" into the leader, which means we get stuff like this:
> >
> >     Heap Blocks: exact=102 lossy=10995
> >     Worker 0:  actual time=50.559..209.773 rows=215253 loops=1
> >        Heap Blocks: exact=207 lossy=19354
> >     Worker 1:  actual time=50.543..211.387 rows=162934 loops=1
> >        Heap Blocks: exact=161 lossy=14636
> >
> > I think this is wrong / confusing, and inconsistent with what we do for
> > other nodes.
>
> Are you able to share which other nodes that you mean here?

I did the analysis on this and out of the node types that have
parallel instrumentation (per ExecParallelRetrieveInstrumentation()),
Parallel Hash is the only node that does anything different from the
others. Looking at the loop inside show_hash_info(), you can see it
takes the Max() of each property. There's some discussion in [1] about
why this came about. In particular [2].

I see no reason to copy the odd one out here, so I'm planning on going
ahead with the patch that has Bitmap Heap Scan copy what the majority
of other nodes do. I think we should consider aligning Parallel Hash
with the other Parallel node behaviour.

I've attached the (roughly done) schema and queries I used to obtain
the plans to do this analysis.

David

[1] https://www.postgresql.org/message-id/flat/20200323165059.GA24950%40alvherre.pgsql
[2] https://www.postgresql.org/message-id/31321.1586549487%40sss.pgh.pa.us

--000000000000e9a499061cc51749
Content-Type: text/plain; charset="US-ASCII"; 
	name="explain_analyze_parallel_stats_analysis.txt"
Content-Disposition: attachment; 
	filename="explain_analyze_parallel_stats_analysis.txt"
Content-Transfer-Encoding: base64
Content-ID: <f_lydmwkrj0>
X-Attachment-Id: f_lydmwkrj0
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==
--000000000000e9a499061cc51749--