Обсуждение: Better performance possible for a pathological query?
Hi,
I've seen a couple of bad queries go through one instance and I'm wondering whether there's something simple that can be done to help.
Not running the query in the first place is what I am looking to do ultimately but in the meantime, I'm interested in understanding more about the plan below.
The query itself is very simple: a primary key lookup on a 1.5x10^7 rows. The issue is that we are looking up over 11,000 primary keys at once, causing the db to consume a lot of CPU.
Where I'm not sure I follow, is the discrepancy between the planned and actual rows.
Bitmap Heap Scan on dim_context c (cost=6923.33..11762.31 rows=1 width=329) (actual time=17128.121..22031.783 rows=10858 loops=1)
Would a sequential scan be more beneficial? The table itself is about 10GB on a 64GB box, 30% of these 10GB are buffered in RAM from what I can tell.
Thanks for your help,
Alexis
Here are the full details.
explain (analyze, buffers)
SELECT c.key,
c.x_id,
c.tags,
c.source_type_id,
x.api_key
FROM dim_context c
join x on c.x_id = x.id
WHERE c.key = ANY (ARRAY[15368196, (11,000 other keys)])
AND ((c.x_id = 1 AND c.tags @> ARRAY[E'blah']))
Here is the plan, abridged
Nested Loop (cost=6923.33..11770.59 rows=1 width=362) (actual time=17128.188..22109.283 rows=10858 loops=1)
Buffers: shared hit=83494
-> Bitmap Heap Scan on dim_context c (cost=6923.33..11762.31 rows=1 width=329) (actual time=17128.121..22031.783 rows=10858 loops=1)
Recheck Cond: ((tags @> '{blah}'::text[]) AND (x_id = 1))
Filter: (key = ANY ('{15368196,(a lot more keys here)}'::integer[]))
Buffers: shared hit=50919
-> BitmapAnd (cost=6923.33..6923.33 rows=269 width=0) (actual time=132.910..132.910 rows=0 loops=1)
Buffers: shared hit=1342
-> Bitmap Index Scan on dim_context_tags_idx (cost=0.00..1149.61 rows=15891 width=0) (actual time=64.614..64.614 rows=264777 loops=1)
Index Cond: (tags @> '{blah}'::text[])
Buffers: shared hit=401
-> Bitmap Index Scan on dim_context_x_id_source_type_id_idx (cost=0.00..5773.47 rows=268667 width=0) (actual time=54.648..54.648 rows=267659 loops=1)
Index Cond: (x_id = 1)
Buffers: shared hit=941
-> Index Scan using x_pkey on x (cost=0.00..8.27 rows=1 width=37) (actual time=0.003..0.004 rows=1 loops=10858)
Index Cond: (x.id = 1)
Buffers: shared hit=32575
Total runtime: 22117.417 ms
And here are the stats
attname | null_frac | avg_width | n_distinct | correlation
----------------+-----------+-----------+------------+-------------
key | 0 | 4 | -1 | 0.999558
x_id | 0 | 4 | 1498 | 0.351316
h_id | 0.05632 | 4 | 116570 | 0.653092
tags | 0.0544567 | 284 | 454877 | -0.169626
source_type_id | 0 | 4 | 23 | 0.39552
handle | 0 | 248 | -1 | 0.272456
created | 0 | 8 | -0.645231 | 0.999559
modified | 0 | 8 | -0.645231 | 0.999559
=?UTF-8?B?QWxleGlzIEzDqi1RdcO0Yw==?= <alq@datadoghq.com> writes:
> The query itself is very simple: a primary key lookup on a 1.5x10^7 rows.
> The issue is that we are looking up over 11,000 primary keys at once,
> causing the db to consume a lot of CPU.
It looks like most of the runtime is probably going into checking the
c.key = ANY (ARRAY[...]) construct. PG isn't especially smart about that
if it fails to optimize the construct into an index operation --- I think
it's just searching the array linearly for each row meeting the other
restrictions on c.
You could try writing the test like this:
c.key = ANY (VALUES (1), (17), (42), ...)
to see if the sub-select code path gives better results than the array
code path. In a quick check it looked like this might produce a hash
join, which seemed promising anyway.
regards, tom lane
On Wed, Aug 7, 2013 at 12:07 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Alexis Lê-Quôc <alq@datadoghq.com> writes:It looks like most of the runtime is probably going into checking the
> The query itself is very simple: a primary key lookup on a 1.5x10^7 rows.
> The issue is that we are looking up over 11,000 primary keys at once,
> causing the db to consume a lot of CPU.
c.key = ANY (ARRAY[...]) construct. PG isn't especially smart about that
if it fails to optimize the construct into an index operation --- I think
it's just searching the array linearly for each row meeting the other
restrictions on c.
You could try writing the test like this:
c.key = ANY (VALUES (1), (17), (42), ...)
to see if the sub-select code path gives better results than the array
code path. In a quick check it looked like this might produce a hash
join, which seemed promising anyway.
regards, tom lane
Thank you very much Tom, your suggestion is spot on. Runtime decreased 100-fold, from 20s to 200ms with a simple search-and-replace.
Here's the updated plan for the record.
Nested Loop (cost=168.22..2116.29 rows=148 width=362) (actual time=22.134..256.531 rows=10858 loops=1)
Buffers: shared hit=44967
-> Index Scan using x_pkey on x (cost=0.00..8.27 rows=1 width=37) (actual time=0.071..0.073 rows=1 loops=1)
Index Cond: (id = 1)
Buffers: shared hit=4
-> Nested Loop (cost=168.22..2106.54 rows=148 width=329) (actual time=22.060..242.406 rows=10858 loops=1)
Buffers: shared hit=44963
-> HashAggregate (cost=168.22..170.22 rows=200 width=4) (actual time=21.529..32.820 rows=11215 loops=1)
-> Values Scan on "*VALUES*" (cost=0.00..140.19 rows=11215 width=4) (actual time=0.005..9.527 rows=11215 loops=1)
-> Index Scan using dim_context_pkey on dim_context c (cost=0.00..9.67 rows=1 width=329) (actual time=0.015..0.016 rows=1 loops=11215)
Index Cond: (c.key = "*VALUES*".column1)
Filter: ((c.tags @> '{blah}'::text[]) AND (c.org_id = 1))
Buffers: shared hit=44963
Total runtime: 263.639 ms