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Query much faster with enable_seqscan=0

From: Ogden <lists(at)darkstatic(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Query much faster with enable_seqscan=0
Date: 2010-09-21 17:32:01
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance

I have received some help from the IRC channel, however, the problem still exists. When running the following query with enable_seqscan set to 0, it takes less than a second, whereas with it set to 1, the query returns in 14 seconds. The machine itself has 8GB Ram and is running PostgreSQL 9.0 on Debian Lenny. The database size is about 7GB. 

SELECT, tr.sid
            test_registration tr,
            INNER JOIN test_registration_result r on ( = r.test_registration_id)
            GROUP BY, tr.sid

demo=# \d test_registration
                       Table "public.test_registration"
         Column         |            Type             |       Modifiers        
 id                     | uuid                        | not null
 sid                    | character varying(36)       | not null
 created_date           | timestamp without time zone | not null default now()
 modified_date          | timestamp without time zone | not null
 test_administration_id | uuid                        | not null
 teacher_number         | character varying(15)       | 
 test_version_id        | uuid                        | 
    "test_registration_pkey" PRIMARY KEY, btree (id)
    "test_registration_sid_key" UNIQUE, btree (sid, test_administration_id)
    "test_registration_teacher" btree (teacher_number)
    "test_registration_test_id" btree (test_administration_id)

demo=# \d test_registration_result
         Table "public.test_registration_result"
        Column        |         Type          | Modifiers 
 answer               | character varying(15) | 
 question_id          | uuid                  | not null
 score                | double precision      | 
 test_registration_id | uuid                  | not null
    "test_registration_result_pkey" PRIMARY KEY, btree (question_id, test_registration_id)
    "test_registration_result_answer" btree (test_registration_id, answer, score)
    "test_registration_result_test" btree (test_registration_id)

Explain Analyze:

 HashAggregate  (cost=951169.97..951198.37 rows=2840 width=25) (actual time=14669.039..14669.843 rows=2972 loops=1)
   ->  Hash Join  (cost=2988.07..939924.85 rows=2249024 width=25) (actual time=551.464..14400.061 rows=638980 loops=1)
         Hash Cond: (r.test_registration_id =
         ->  Seq Scan on test_registration_result r  (cost=0.00..681946.72 rows=37199972 width=16) (actual time=0.015..6073.101 rows=37198734 loops=1)
         ->  Hash  (cost=2952.57..2952.57 rows=2840 width=25) (actual time=2.516..2.516 rows=2972 loops=1)
               Buckets: 1024  Batches: 1  Memory Usage: 160kB
               ->  Bitmap Heap Scan on test_registration tr  (cost=44.29..2952.57 rows=2840 width=25) (actual time=0.528..1.458 rows=2972 loops=1)
                     Recheck Cond: (test_administration_id = 'e26a165a-c19f-11df-be2f-778af560e5a2'::uuid)
                     ->  Bitmap Index Scan on test_registration_test_administration_id  (cost=0.00..43.58 rows=2840 width=0) (actual time=0.507..0.507 rows=2972 loops=1)
                           Index Cond: (test_administration_id = 'e26a165a-c19f-11df-be2f-778af560e5a2'::uuid)
 Total runtime: 14670.337 ms
(11 rows)

real	0m14.698s
user	0m0.000s
sys	0m0.008s

With "set enable_seqscan=0;"

                                                                                     QUERY PLAN                                                                                      
 HashAggregate  (cost=1225400.19..1225428.59 rows=2840 width=25) (actual time=748.397..749.160 rows=2972 loops=1)
   ->  Nested Loop  (cost=0.00..1214155.07 rows=2249024 width=25) (actual time=0.107..465.165 rows=638980 loops=1)
         ->  Index Scan using test_registration_test_administration_id on test_registration tr  (cost=0.00..4413.96 rows=2840 width=25) (actual time=0.050..1.610 rows=2972 loops=1)
               Index Cond: (test_administration_id = 'e26a165a-c19f-11df-be2f-778af560e5a2'::uuid)
         ->  Index Scan using test_registration_result_answer on test_registration_result r  (cost=0.00..416.07 rows=792 width=16) (actual time=0.019..0.106 rows=215 loops=2972)
               Index Cond: (r.test_registration_id =
 Total runtime: 749.745 ms
(7 rows)

real	0m0.759s
user	0m0.008s
sys	0m0.000s

The following parameters are changed in postgresql.conf and I have routinely vacuum analyzed the tables and database:

shared_buffers = 2048MB 
work_mem = 8MB
maintenance_work_mem = 256MB 
wal_buffers = 640kB
random_page_cost = 4.0  
effective_cache_size = 7000MB
default_statistics_target = 200 

free -m:
             total       used       free     shared    buffers     cached
Mem:          8003       7849        153          0         25       7555
-/+ buffers/cache:        268       7735
Swap:         7640          0       7639

Any help would be appreciated. Thank you very much. 



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Next:From: Kenneth MarshallDate: 2010-09-21 18:06:18
Subject: Re: Query much faster with enable_seqscan=0
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Subject: Re: Auto ANALYZE criteria

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