Re: [PATCH] Incremental sort (was: PoC: Partial sort) - Mailing list pgsql-hackers

On Sat, Mar 28, 2020 at 10:47:49PM -0400, James Coleman wrote:
>On Sat, Mar 28, 2020 at 6:59 PM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>>
>> Hi,
>>
>> Attached is my take on simplification of the useful pathkeyes thing. It
>> keeps the function, but it truncates query_pathkeys to only members with
>> EC members in the relation. I think that's essentially the optimization
>> you've proposed.
>
>Thanks. I've included that in the patch series in this email (as a
>separate patch) with a few additional comments.
>

Thanks.

>I've also noticed that the enabled_incrementalsort check in
>generate_useful_gather_paths seemed broken, because it returned us out
>of the function before creating either a plain gather merge (if
>already sorted) or an explicit sort path. I've included a patch that
>moves it to the if block that actually builds the incremental sort
>path.
>

Hmmm, that's probably right.

The reason why the GUC check was right after generate_gather_paths is
that the intent to disable all the useful-pathkeys business, essentially
reducing it back to plain generate_gather_paths.

But I think you're right that's wrong, because it might lead to strange
behavior when the GUC switches between plans without any incremental
sort nodes - setting it to 'true' might end up picking GM on top of
plain Sort, for example.

>>
>> ...
>>
>> 1) Missing worker identification (Worker #).
>
>Fixed.
>
>> 2) Missing method for workers (we have it for the leader, though).
>
>Fixed. Since we can't have pointers in the parallel shared memory
>space, we can't store the sort methods used in a list. To accomplish
>the same goal, I've assigned the TuplesortMethod enum entries uique
>bit positions, and store the methods used in a bitmask.
>

OK, makes sense.

>> 3) I'm not sure why the lable is "Methods" instead of "Sort Method", and
>> why it's in parenthesis.
>
>I've removed the parentheses. It's labeled "Methods" since there can
>be more than one (different batches could use different methods). I've
>updated this to properly use singular/plural depending on the number
>of methods used.
>

I'm a bit confused. How do I know which batch used which method? Is it
actually worth printing in explain analyze? Maybe only print it in the
verbose mode?

>> 4) Not sure having two lines for each worker is a great idea.
>
>I've left these in for now because the lines are already very long
>(much, much longer than the worker lines in a standard sort node).
>This is largely because we're trying to summarize many sort batches,
>while standard sort nodes only have to give the exact stats from a
>single batch.
>
>See the example output later in the email.
>

OK

>> 5) I'd probably prefer having multiple labels for avg/max memory values,
>> instead of (avg) and (max) notes. Also, I think we use "peak" in this
>> context instead of "max".
>
>Updated.
>

OK

>Here's the current output:
>
> Limit  (cost=1887419.20..1889547.68 rows=10000 width=8) (actual
>time=13218.403..13222.519 rows=10000 loo
>ps=1)
>   ->  Gather Merge  (cost=1887419.20..19624748.03 rows=83333360
>width=8) (actual time=13218.401..13229.7
>50 rows=10000 loops=1)
>         Workers Planned: 2
>         Workers Launched: 2
>         ->  Incremental Sort  (cost=1886419.17..10005010.55
>rows=41666680 width=8) (actual time=13208.00
>4..13208.586 rows=4425 loops=3)
>               Sort Key: a, b
>               Presorted Key: a
>               Full-sort Groups: 1 Sort Method: quicksort Memory:
>avg=28kB peak=28kB
>               Presorted Groups: 1 Sort Method: top-N heapsort Memory:
>avg=1681kB peak=1681kB
>               Worker 0:  Full-sort Groups: 1 Sort Method: quicksort
>Memory: avg=28kB peak=28kB
>                 Presorted Groups: 1 Sort Method: top-N heapsort
>Memory: avg=1680kB peak=1680kB
>               Worker 1:  Full-sort Groups: 1 Sort Method: quicksort
>Memory: avg=28kB peak=28kB
>                 Presorted Groups: 1 Sort Method: top-N heapsort
>Memory: avg=1682kB peak=1682kB
>               ->  Parallel Index Scan using index_s_a on s
>(cost=0.57..4967182.06 rows=41666680 width=8
>) (actual time=0.455..11730.878 rows=6666668 loops=3)
>
>James

Looks reasonable. Did you try it in other output formats - json/yaml?


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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