Re: PoC: Partial sort

From: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
To: David Rowley <dgrowleyml(at)gmail(dot)com>
Cc: Marti Raudsepp <marti(at)juffo(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com>, Andreas Karlsson <andreas(at)proxel(dot)se>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Andres Freund <andres(at)2ndquadrant(dot)com>, Martijn van Oosterhout <kleptog(at)svana(dot)org>
Subject: Re: PoC: Partial sort
Date: 2014-09-13 09:04:00
Message-ID: CAPpHfdv49Lc5HKHMsXskQJRdZ0ULanPD4qXz=dFv0OaAGCCWnw@mail.gmail.com
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On Tue, Aug 19, 2014 at 2:02 PM, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> Here's a few notes from reading over the code:
>
> * pathkeys.c
>
> EquivalenceMember *member = (EquivalenceMember *)
> lfirst(list_head(key->pk_eclass->ec_members));
>
> You can use linitial() instead of lfirst(list_head()). The same thing
> occurs in costsize.c
>

Fixed.

> * pathkeys.c
>
> The following fragment:
>
> n = pathkeys_common(root->query_pathkeys, pathkeys);
>
> if (n != 0)
> {
> /* It's useful ... or at least the first N keys are */
> return n;
> }
>
> return 0; /* path ordering not useful */
> }
>
> Could just read:
>
> /* return the number of path keys in common, or 0 if there are none */
> return pathkeys_common(root->query_pathkeys, pathkeys);
>

Fixed.

> * execnodes.h
>
> In struct SortState, some new fields don't have a comment.
>

Fixed.

> I've also thrown a few different workloads at the patch and I'm very
> impressed with most of the results. Especially when LIMIT is used, however
> I've found a regression case which I thought I should highlight, but for
> now I can't quite see what could be done to fix it.
>
> create table a (x int not null, y int not null);
> insert into a select x.x,y.y from generate_series(1,1000000) x(x) cross
> join generate_series(1,10) y(y);
>
> Patched:
> explain analyze select x,y from a where x+0=1 order by x,y limit 10;
> QUERY PLAN
>
> ------------------------------------------------------------------------------------------------------------------------------------
> Limit (cost=92.42..163.21 rows=10 width=8) (actual
> time=6239.426..6239.429 rows=10 loops=1)
> -> Partial sort (cost=92.42..354064.37 rows=50000 width=8) (actual
> time=6239.406..6239.407 rows=10 loops=1)
> Sort Key: x, y
> Presorted Key: x
> Sort Method: quicksort Memory: 25kB
> -> Index Scan using a_x_idx on a (cost=0.44..353939.13
> rows=50000 width=8) (actual time=0.059..6239.319 rows=10 loops=1)
> Filter: ((x + 0) = 1)
> Rows Removed by Filter: 9999990
> Planning time: 0.212 ms
> Execution time: 6239.505 ms
> (10 rows)
>
>
> Time: 6241.220 ms
>
> Unpatched:
> explain analyze select x,y from a where x+0=1 order by x,y limit 10;
> QUERY PLAN
>
> --------------------------------------------------------------------------------------------------------------------
> Limit (cost=195328.26..195328.28 rows=10 width=8) (actual
> time=3077.759..3077.761 rows=10 loops=1)
> -> Sort (cost=195328.26..195453.26 rows=50000 width=8) (actual
> time=3077.757..3077.757 rows=10 loops=1)
> Sort Key: x, y
> Sort Method: quicksort Memory: 25kB
> -> Seq Scan on a (cost=0.00..194247.77 rows=50000 width=8)
> (actual time=0.018..3077.705 rows=10 loops=1)
> Filter: ((x + 0) = 1)
> Rows Removed by Filter: 9999990
> Planning time: 0.510 ms
> Execution time: 3077.837 ms
> (9 rows)
>
>
> Time: 3080.201 ms
>
> As you can see, the patched version performs an index scan in order to get
> the partially sorted results, but it does end up quite a bit slower than
> the seqscan/sort that the unpatched master performs. I'm not quite sure how
> realistic the x+0 = 1 WHERE clause is, but perhaps the same would happen if
> something like x+y = 1 was performed too.... After a bit more analysis on
> this, I see that if I change the 50k estimate to 10 in the debugger that
> the num_groups is properly estimated at 1 and it then performs the seq scan
> instead. So it looks like the costings of the patch are not to blame here.
> (The 50k row estimate comes from rel tuples / DEFAULT_NUM_DISTINCT)
>

Yes, the error comes from assumption of 50k row estimate. I've checked
similar example when estimate is fine.

create table b as (select x.x,y.y,x.x z from generate_series(1,1000000)
x(x) cross join generate_series(1,10) y(y));
create index b_x_idx on b(x);
analyze b;

There is column z which is both not in index and not in "order by" clause.
If we replace "x+0=1" with "z=1" optimizer didn't decide to use partial
sort.

explain analyze select x,y,z from b where z=1 order by x,y limit 10;
QUERY PLAN
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Limit (cost=179056.59..179056.61 rows=10 width=12) (actual
time=1072.498..1072.500 rows=10 loops=1)
-> Sort (cost=179056.59..179056.63 rows=18 width=12) (actual
time=1072.495..1072.495 rows=10 loops=1)
Sort Key: x, y
Sort Method: quicksort Memory: 25kB
-> Seq Scan on b (cost=0.00..179056.21 rows=18 width=12) (actual
time=0.020..1072.454 rows=10 loops=1)
Filter: (z = 1)
Rows Removed by Filter: 9999990
Planning time: 0.501 ms
Execution time: 1072.555 ms
(9 rows)

If we event force optimizer to use partial sort then cost estimation will
be fine.

set enable_seqscan = off;
explain analyze select x,y,z from b where z=1 order by x,y limit 10;
QUERY PLAN
──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
Limit (cost=169374.43..263471.04 rows=10 width=12) (actual
time=2237.082..2237.083 rows=10 loops=1)
-> Partial sort (cost=169374.43..338748.34 rows=18 width=12) (actual
time=2237.082..2237.083 rows=10 loops=1)
Sort Key: x, y
Presorted Key: x
Sort Method: quicksort Memory: 25kB
-> Index Scan using b_x_idx on b (cost=0.43..338748.13 rows=18
width=12) (actual time=0.047..2237.062 rows=10 loops=1)
Filter: (z = 1)
Rows Removed by Filter: 9999990
Planning time: 0.089 ms
Execution time: 2237.133 ms
(10 rows)

AFAICS wrong selectivity estimations are general problem which cause
optimizer failures. But in your example "x+y=1" if expression index on
"x+y" would exist then statistics over "x+y" will be collected. So, in case
of expression index estimation will be fine.

------
With best regards,
Alexander Korotkov.

Attachment Content-Type Size
partial-sort-basic-2.patch application/octet-stream 73.3 KB
partial-sort-merge-2.patch application/octet-stream 16.4 KB

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