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

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..9808.449rows=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?

> 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.

> My TODO for this patch is this:
>
> - review the costing (I think the estimates are OK, but I recall I
>    haven't been entirely happy with how it's broken into functions.)
>
> - review the tuplesort changes (the memory contexts etc.)
>
> - do more testing of performance impact on planning

Sounds good.

Thanks,
James