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Re: Very poor performance

From: Mark Rostron <mrostron(at)ql2(dot)com>
To: Aaron Burnett <aburnett(at)bzzagent(dot)com>, "pgsql-performance(at)postgresql(dot)org"<pgsql-performance(at)postgresql(dot)org>
Subject: Re: Very poor performance
Date: 2010-08-17 01:51:26
Message-ID: FD020D3E50E7FA479567872E5F5F31E30459E0ECF1@ex01.corp.ql2.com (view raw or flat)
Thread:
Lists: pgsql-performance
This is weird - is there a particular combination of memberid/answered in answerselectindex that has a very high rowcount?

First change I would suggest looking into would be to try changing sub-query logic to check existence and limit the result set of the sub-query to a single row

Select distinct(m.id)
>From member m
Where exists (
      Select 1
      From answerselectinstance a
      Where a.member_id = m.id
      And a.answerid between 127443 and 127448
      Limit 1
)


If member.id is a primary key, you can eliminate the "distinct" i.e. the sort.


Second would be to build a partial index on answersselectindex to index only the memberid's you are interested in:

"Create index <new_index_name> on answersselectindex(memberid) where answerid between 127443 and 127448"



Mr



From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of Aaron Burnett
Sent: Monday, August 16, 2010 6:07 PM
To: pgsql-performance(at)postgresql(dot)org
Subject: [PERFORM] Very poor performance



Hi,

I'm hoping someone can offer some help here. The query and explain analyze and table layout are below and attached in a text file if the formatting is bad.

The query is part of a bigger query that our front end runs. This is the part that takes forever (84 minutes in this case) to finish and more often than not the front end times out. The table (answerselectinstance) has 168664317 rows while the member table has 626435 rows.

Postgres Version 8.25
CentOs 5.2
16 Gig RAM
192MB work_mem  (increasing to 400MB didn't change the outcome)
very light use on this server, it ais a slave to a slony replicated master/slave setup.

Again, apologies if the formatting got munged, the attached text file has the same info.

Thanking you in advance for any help and suggestions.

Aaron

explain analyze select distinct(id) from member  where id in (select memberid from answerselectinstance where   nswerid = 127443  OR  answerid = 127444  OR  answerid = 127445  OR  answerid = 127446  OR  answerid = 127447  OR  answerid = 127448   ) ;

LOG:  duration: 5076038.709 ms  statement: explain analyze select distinct(id) from member  where id in (select memberid from answerselectinstance where   answerid = 127443  OR  answerid = 127444  OR  answerid = 127445  OR  answerid = 127446  OR  answerid = 127447  OR  answerid = 127448   ) ;
                                                                              QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=101357.24..101357.28 rows=9 width=4) (actual time=5075511.974..5075911.077 rows=143520 loops=1)
   ->  Sort  (cost=101357.24..101357.26 rows=9 width=4) (actual time=5075511.971..5075644.323 rows=143520 loops=1)
         Sort Key: member.id
         ->  Nested Loop IN Join  (cost=0.00..101357.10 rows=9 width=4) (actual time=19.867..5075122.724 rows=143520 loops=1)
               ->  Seq Scan on member  (cost=0.00..78157.65 rows=626265 width=4) (actual time=3.338..2003.582 rows=626410 loops=1)
               ->  Index Scan using asi_memberid_idx on answerselectinstance  (cost=0.00..444.46 rows=9 width=4) (actual time=8.096..8.096 rows=0 loops=626410)
                     Index Cond: (member.id = answerselectinstance.memberid)
                     Filter: ((answerid = 127443) OR (answerid = 127444) OR (answerid = 127445) OR (answerid = 127446) OR (answerid = 127447) OR (answerid = 127448))
 Total runtime: 5076034.203 ms
(9 rows)

     Column     |            Type             |                         Modifiers
----------------+-----------------------------+------------------------------------------------------------
 memberid       | integer                     | not null
 answerid       | integer                     | not null
 taskinstanceid | integer                     | not null default 0
 created        | timestamp without time zone | default "timestamp"('now'::text)
 id             | integer                     | not null default nextval(('"asi_id_seq"'::text)::regclass)
Indexes:
    "asi_pkey" PRIMARY KEY, btree (id)
    "asi_answerid_idx" btree (answerid)
    "asi_memberid_idx" btree (memberid)
    "asi_taskinstanceid_idx" btree (taskinstanceid)
Triggers:
    _bzzprod_cluster_denyaccess_301 BEFORE INSERT OR DELETE OR UPDATE ON answerselectinstance FOR EACH ROW EXECUTE PROCEDURE _bzzprod_cluster.denyaccess('_bzzprod_cluster')

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