Date: Mon, 30 Mar 2020 14:24:26 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: James Coleman <jtc331@gmail.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>,
	Michael Paquier <michael@paquier.xyz>,
	Rafia Sabih <rafia.pghackers@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>, Simon Riggs <simon@2ndquadrant.com>,
	Shaun Thomas <shaun.thomas@2ndquadrant.com>,
	Dmitry Dolgov <9erthalion6@gmail.com>,
	Alexander Korotkov <a.korotkov@postgrespro.ru>,
	Tom Lane <tgl@sss.pgh.pa.us>,
	PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>,
	Andreas Karlsson <andreas@proxel.se>
Subject: Re: [PATCH] Incremental sort (was: PoC: Partial sort)
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On Sun, Mar 29, 2020 at 10:16:53PM -0400, James Coleman wrote:
>On Sun, Mar 29, 2020 at 9:44 PM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>>
>> Hi,
>>
>> Attached is a slightly reorganized patch series. I've merged the fixes
>> into the appropriate matches, and I've also combined the two patches
>> adding incremental sort paths to additional places in planner.
>>
>> A couple more comments:
>>
>>
>> 1) I think the GUC documentation in src/sgml/config.sgml is a bit too
>> detailed, compared to the other enable_* GUCs. I wonder if there's a
>> better place where to move the details. What about adding some examples
>> and explanation to perform.sgml?
>
>I'll take a look at that and include in a patch series tomorrow.
>
>> 2) Looking at the explain output, the verbose mode looks like this:
>>
>> test=# explain (verbose, analyze) select a from t order by a, b, c;
>>                                                                            QUERY PLAN
>> --------------------------------------------------------------------------------------------------------------------------------------------------------------
>>   Gather Merge  (cost=66.31..816072.71 rows=8333226 width=24) (actual time=4.787..20092.555 rows=10000000 loops=1)
>>     Output: a, b, c
>>     Workers Planned: 2
>>     Workers Launched: 2
>>     ->  Incremental Sort  (cost=66.28..729200.36 rows=4166613 width=24) (actual time=1.308..14021.575 rows=3333333 loops=3)
>>           Output: a, b, c
>>           Sort Key: t.a, t.b, t.c
>>           Presorted Key: t.a, t.b
>>           Full-sort Groups: 4169 Sort Method: quicksort Memory: avg=30kB peak=30kB
>>           Presorted Groups: 4144 Sort Method: quicksort Memory: avg=128kB peak=138kB
>>           Worker 0:  actual time=0.766..16122.368 rows=3841573 loops=1
>>   Full-sort Groups: 6871 Sort Method: quicksort Memory: avg=30kB peak=30kB
>>             Presorted Groups: 6823 Sort Method: quicksort Memory: avg=132kB peak=141kB
>>           Worker 1:  actual time=1.986..16189.831 rows=3845490 loops=1
>>   Full-sort Groups: 6874 Sort Method: quicksort Memory: avg=30kB peak=30kB
>>             Presorted Groups: 6847 Sort Method: quicksort Memory: avg=130kB peak=139kB
>>           ->  Parallel Index Scan using t_a_b_idx on public.t  (cost=0.43..382365.92 rows=4166613 width=24) (actual time=0.040..9808.449 rows=3333333 loops=3)
>>                 Output: a, b, c
>>                 Worker 0:  actual time=0.048..11275.178 rows=3841573 loops=1
>>                 Worker 1:  actual time=0.041..11314.133 rows=3845490 loops=1
>>   Planning Time: 0.166 ms
>>   Execution Time: 25135.029 ms
>> (22 rows)
>>
>> There seems to be missing indentation for the first line of worker info.
>
>Working on that too.
>
>> I'm still not quite convinced we should be printing two lines - I know
>> you mentioned the lines might be too long, but see how long the other
>> lines may get ...
>
>All right, I give in :)
>
>Do you think non-workers (both the leader and non-parallel plans)
>should also move to one line?
>

I think we should use the same formatting for both cases, so yes.

FWIW I forgot to mention I tweaked the INSTRUMENT_SORT_GROUP macro a
bit, by moving the if condition in it. That makes the calls easier.

>> 3) I see the new nodes (plan state, ...) have "presortedCols" which does
>> not indicate it's a "number of". I think we usually prefix names of such
>> fields "n" or "num". What about "nPresortedCols"? (Nitpicking, I know.)
>
>I can fix this too.
>
>Also I noticed a few compiler warnings I'll fixup in tomorrow's reply.
>

OK

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services