Re: Hash Anti Join performance degradation

2011/5/24 panam <panam@gmx.net>:
> Hi,
>
> In my DB, there is a large table containing messages and one that contains
> message boxes.
> Messages are assigned to boxes via a child parent link m->b.
> In order to obtain the last message for a specific box, I use the following
> SQL:
>
> SELECT m1.id FROM message m1 LEFT JOIN message m2 ON (m1.box_id = m2.box_id
> AND m1.id < m2.id) WHERE m2.id IS NULL AND m1.box_id = id;
>
> This worked quite well for a long time. But now, suddenly the performance of
> this query drastically degraded as new messages were added.
> If these new messages are removed again, everything is back to normal. If
> other messages are removed instead, the problem remains, so it does not seem
> to be a memory issue. I fear I have difficulties to understand what is going
> wrong here.

We need more information here. The case is in fact interesting.
What's the PostgreSQL version, and did you have log of vacuum and
checkpoint activity ? (no vacuum full/cluster or such thing running ?)

Obvisouly, Craig suggestion to read
http://wiki.postgresql.org/wiki/SlowQueryQuestions is relevant as it
helps to have all common information required to analyze the issue.

>
> This is the query plan when everything is fine:
>
> "Seq Scan on public.box this_  (cost=0.00..10467236.32 rows=128 width=696)
> (actual time=0.169..7683.978 rows=128 loops=1)"
> "  Output: this_.id, this_.login, (SubPlan 1)"
> "  Buffers: shared hit=188413 read=94635 written=135, temp read=22530
> written=22374"
> "  SubPlan 1"
> "    ->  Hash Anti Join  (cost=41323.25..81775.25 rows=20427 width=8)
> (actual time=59.571..59.877 rows=1 loops=128)"
> "          Output: m1.id"
> "          Hash Cond: (m1.box_id = m2.box_id)"
> "          Join Filter: (m1.id < m2.id)"
> "          Buffers: shared hit=188412 read=94633 written=135, temp
> read=22530 written=22374"
> "          ->  Bitmap Heap Scan on public.message m1  (cost=577.97..40212.28
> rows=30640 width=16) (actual time=3.152..9.514 rows=17982 loops=128)"
> "                Output: m1.id, m1.box_id"
> "                Recheck Cond: (m1.box_id = $0)"
> "                Buffers: shared hit=131993 read=9550 written=23"
> "                ->  Bitmap Index Scan on "message_box_Idx"
> (cost=0.00..570.31 rows=30640 width=0) (actual time=2.840..2.840 rows=18193
> loops=128)"
> "                      Index Cond: (m1.box_id = $0)"
> "                      Buffers: shared hit=314 read=6433 written=23"
> "          ->  Hash  (cost=40212.28..40212.28 rows=30640 width=16) (actual
> time=26.840..26.840 rows=20014 loops=115)"
> "                Output: m2.box_id, m2.id"
> "                Buckets: 4096  Batches: 4 (originally 2)  Memory Usage:
> 5444kB"
> "                Buffers: shared hit=56419 read=85083 written=112, temp
> written=7767"
> "                ->  Bitmap Heap Scan on public.message m2
> (cost=577.97..40212.28 rows=30640 width=16) (actual time=2.419..20.007
> rows=20014 loops=115)"
> "                      Output: m2.box_id, m2.id"
> "                      Recheck Cond: (m2.box_id = $0)"
> "                      Buffers: shared hit=56419 read=85083 written=112"
> "                      ->  Bitmap Index Scan on "message_box_Idx"
> (cost=0.00..570.31 rows=30640 width=0) (actual time=2.166..2.166 rows=20249
> loops=115)"
> "                            Index Cond: (m2.box_id = $0)"
> "                            Buffers: shared hit=6708"
> "Total runtime: 7685.202 ms"
>
> This is the plan when the query gets sluggish:
>
> "Seq Scan on public.box this_  (cost=0.00..10467236.32 rows=128 width=696)
> (actual time=0.262..179333.086 rows=128 loops=1)"
> "  Output: this_.id, this_.login, (SubPlan 1)"
> "  Buffers: shared hit=189065 read=93983 written=10, temp read=22668
> written=22512"
> "  SubPlan 1"
> "    ->  Hash Anti Join  (cost=41323.25..81775.25 rows=20427 width=8)
> (actual time=1264.700..1400.886 rows=1 loops=128)"
> "          Output: m1.id"
> "          Hash Cond: (m1.box_id = m2.box_id)"
> "          Join Filter: (m1.id < m2.id)"
> "          Buffers: shared hit=189064 read=93981 written=10, temp read=22668
> written=22512"
> "          ->  Bitmap Heap Scan on public.message m1  (cost=577.97..40212.28
> rows=30640 width=16) (actual time=3.109..9.850 rows=18060 loops=128)"
> "                Output: m1.id, m1.box_id"
> "                Recheck Cond: (m1.box_id = $0)"
> "                Buffers: shared hit=132095 read=9448"
> "                ->  Bitmap Index Scan on "message_box_Idx"
> (cost=0.00..570.31 rows=30640 width=0) (actual time=2.867..2.867 rows=18193
> loops=128)"
> "                      Index Cond: (m1.box_id = $0)"
> "                      Buffers: shared hit=312 read=6435"
> "          ->  Hash  (cost=40212.28..40212.28 rows=30640 width=16) (actual
> time=27.533..27.533 rows=20102 loops=115)"
> "                Output: m2.box_id, m2.id"
> "                Buckets: 4096  Batches: 4 (originally 2)  Memory Usage:
> 5522kB"
> "                Buffers: shared hit=56969 read=84533 written=10, temp
> written=7811"
> "                ->  Bitmap Heap Scan on public.message m2
> (cost=577.97..40212.28 rows=30640 width=16) (actual time=2.406..20.492
> rows=20102 loops=115)"
> "                      Output: m2.box_id, m2.id"
> "                      Recheck Cond: (m2.box_id = $0)"
> "                      Buffers: shared hit=56969 read=84533 written=10"
> "                      ->  Bitmap Index Scan on "message_box_Idx"
> (cost=0.00..570.31 rows=30640 width=0) (actual time=2.170..2.170 rows=20249
> loops=115)"
> "                            Index Cond: (m2.box_id = $0)"
> "                            Buffers: shared hit=6708"
> "Total runtime: 179334.310 ms"
>
>
> So from my limited experience, the only significant difference I see is that
> the Hash Anti Join takes a lot more time in plan 2, but I do not understand
> why.
> Ideas somebody?
>
> Thanks
> panam
>
>
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Cédric Villemain               2ndQuadrant
http://2ndQuadrant.fr/     PostgreSQL : Expertise, Formation et Support