Horton

Re: Parallel Bitmap Heap Scan reports per-worker stats in EXPLAIN ANALYZE

Donghang Lin <donghanglin@gmail.com>

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From: Donghang Lin <donghanglin@gmail.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Dilip Kumar <dilipbalaut@gmail.com>, David Geier <geidav.pg@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, hlinnaka@iki.fi
Date: 2024-03-27T07:03:08Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Show Parallel Bitmap Heap Scan worker stats in EXPLAIN ANALYZE

  2. Widen lossy and exact page counters for Bitmap Heap Scan

  3. Remove redundant snapshot copying from parallel leader to workers

Attachments

On Mon, Mar 25, 2024 at 2:11 PM Melanie Plageman <melanieplageman@gmail.com>
wrote:
> As for whether or not per-worker stats should be displayed by default
> or only with VERBOSE, it sounds like there are two different
> precedents. I don't have a strong feeling one way or the other.
> Whichever is most consistent.
> Donghang, could you list again which plan nodes and explain options
> always print per-worker stats and which only do with the VERBOSE
> option?

I took a look at explain.c where workers info is printed out.

These works for every parallel aware nodes:
Buffers stats print for workers with VERBOSE and BUFFERS
WAL stats print for workers with VERBOSE and WAL
JIT stats print for workers with VERBOSE and COSTS
Timing print for workers with VERBOSE and TIMING
Rows and loops print for workers with VERBOSE

Some specific nodes:
Sort / Incremental Sort / Hash / HashAggregate / Memorize and Bitmap Heap
Scan (this patch) nodes
always print their specific stats for workers.

> I did some logging and I do see workers with
> counts of lossy/exact not making it into the final count. I haven't
> had time to debug more, but it is worth looking into.

Indeed, rescan overrides previous scan stats in workers.
Attach v5 with v4 plus the fix to aggregate the counts.

Regards,
Donghang Lin
(ServiceNow)