You DB is more than likely cached. You should adjust your page costs to better reflect reality and then the planner can make more accurate estimates and then choose the proper plan.
Cheers, Ken On Tue, Sep 21, 2010 at 12:32:01PM -0500, Ogden wrote: > Hello, > > 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. > > > Query: > SELECT tr.id, tr.sid > FROM > test_registration tr, > INNER JOIN test_registration_result r on (tr.id = > r.test_registration_id) > WHERE. > > tr.test_administration_id='32a22b12-aa21-11df-a606-96551e8f4e4c'::uuid > GROUP BY tr.id, 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 | > Indexes: > "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 > Indexes: > "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 = tr.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;" > > > SET > > 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 = tr.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. > > Ogden -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
