Re: Summary Sort workers Stats in EXPLAIN ANALYZE

I have updated the patch addressing the review comments, but I didn't moved this code block into VERBOSE mode, to keep consistency with `show_incremental_sort_info`:

https://github.com/postgres/postgres/blob/d4ba8b51c76300f06cc23f4d8a41d9f7210c4866/src/backend/commands/explain.c#L2890

Please review, thanks.

________________________________
From: Julien Rouhaud <rjuju123(at)gmail(dot)com>
Sent: Friday, March 25, 2022 17:04
To: Jian Guo <gjian(at)vmware(dot)com>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org <pgsql-hackers(at)lists(dot)postgresql(dot)org>; Zhenghua Lyu <zlyu(at)vmware(dot)com>
Subject: Re: Summary Sort workers Stats in EXPLAIN ANALYZE

⚠ External Email

Hi,

On Thu, Mar 24, 2022 at 07:50:11AM +0000, Jian Guo wrote:
> For a simple demo, with this explain statement：
>
> -- Test sort stats summary
> set force_parallel_mode=on;
> select explain_filter('explain (analyze, summary off, timing off, costs off, format json) select * from tenk1 order by unique1');
>
> Before this patch, we got plan like this:
>
>
> "Node Type": "Sort", +
> "Parent Relationship": "Outer", +
> "Parallel Aware": false, +
> "Async Capable": false, +
> "Actual Rows": 10000, +
> "Actual Loops": 1, +
> "Sort Key": ["unique1"], +
> "Workers": [ +
> { +
> "Worker Number": 0, +
> "Sort Method": "external merge",+
> "Sort Space Used": 2496, +
> "Sort Space Type": "Disk" +
> } +
> ], +

> After this patch, the effected plan is this:
>
> "Node Type": "Sort", +
> "Parent Relationship": "Outer", +
> "Parallel Aware": false, +
> "Async Capable": false, +
> "Actual Rows": N, +
> "Actual Loops": N, +
> "Sort Key": ["unique1"], +
> "Workers planned": N, +
> "Sort Method": "external merge", +
> "Average Sort Space Used": N, +
> "Peak Sort Space Used": N, +
> "Sort Space Type": "Disk", +

I think the idea is interesting, however there are a few problems in the patch.

First, I think that it should only be done in the VERBOSE OFF mode. If you ask
for a VERBOSE output you don't need both the details and the summarized
version.

Other minor problems:

- why (only) emitting the number of workers planned and not the number of
workers launched?
- the textual format is missing details about what the numbers are, which is
particularly obvious since avgSpaceUsed and peakSpaceUsed don't have any unit
or even space between them:

+ "Sort Method: %s %s: " INT64_FORMAT INT64_FORMAT "kB\n",
+ sortMethod, spaceType, avgSpaceUsed, peakSpaceUsed);

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