Re: [PERFORM] Big diference in response time (query plan question)
Hi Dave, Thanks to reply. I run it now in a Postgres 8.1.4 my notebook (win XP) and the performance is really much better: EXPLAIN ANALYZE SELECT Contrato.Id , Min( prog.dtsemeio ) AS DtSemIni , Max( prog.dtsemeio ) AS DtSemFim , Min( prog.dtembarque ) AS DtEmbIni , Max( prog.dtembarque ) AS DtEmbFim , Min( prog.dtentrega ) AS DtEntIni , Max( prog.dtentrega ) AS DtEntFim , COUNT(prog.*) AS QtSem , SUM( CASE WHEN Prog.DtSemeio = '20060814' THEN 1 ELSE 0 END ) AS QtSemAb FROM bvz.Contrato LEFT OUTER JOIN bvz.Prog ON prog.Fk_Contrato = Contrato.Id WHEREContrato.Fk_Clifor = 243 GROUP BY 1; GroupAggregate (cost=2.18..7312.45 rows=42 width=48) (actual time=0.446..13.195 rows=42 loops=1) - Nested Loop Left Join (cost=2.18..7291.22 rows=883 width=48) (actual time=0.103..10.518 rows=1536 loops=1) - Index Scan using pk_contrato on contrato (cost=0.00..100.29 rows=42 width=4) (actual time=0.048..3.163 rows=42 loops=1) Filter: (fk_clifor = 243) - Bitmap Heap Scan on prog (cost=2.18..170.59 rows=50 width=48) (actual time=0.027..0.132 rows=37 loops=42) Recheck Cond: (prog.fk_contrato = outer.id) - Bitmap Index Scan on fki_prog_contrato (cost=0.00..2.18 rows=50 width=0) (actual time=0.018..0.018 rows=37 loops=42) Index Cond: (prog.fk_contrato = outer.id) Total runtime: 13.399 ms Where I can see the current random_page_cost value ? There are some hint about what value I must set ? Thanks in advance. Luiz Dave Dutcher wrote: Well, in this case the queries with LEFT OUTER join and with inner join returns the same result set. I don´t have the sufficient knowledge to affirm , but I suspect that if the query plan used for fk_clifor = 352 and with left outer join is applied for the first query (fk_clifor = 243 with left outer join) we will have a better total runtime. There are some manner to make this test ? It looks like Postgres used a nested loop join for the fast query and a merge join for the slow query. I don't think the left join is causing any problems. On the slower query the cost estimate of the nested loop must have been higher than the cost estimate of the merge join because of more rows. You could try disabling merge joins with the command set enable_mergejoin=false. Then run the explain analyze again to see if it is faster. If it is faster without merge join, then you could try to change your settings to make the planner prefer the nested loop. I'm not sure what the best way to do that is. Maybe you could try reducing the random_page_cost, which should make index scans cheaper. Dave ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Big diference in response time (query plan question)
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Luiz K. Matsumura Well, in this case the queries with LEFT OUTER join and with inner join returns the same result set. I don´t have the sufficient knowledge to affirm , but I suspect that if the query plan used for fk_clifor = 352 and with left outer join is applied for the first query (fk_clifor = 243 with left outer join) we will have a better total runtime. There are some manner to make this test ? It looks like Postgres used a nested loop join for the fast query and a merge join for the slow query. I don't think the left join is causing any problems. On the slower query the cost estimate of the nested loop must have been higher than the cost estimate of the merge join because of more rows. You could try disabling merge joins with the command set enable_mergejoin=false. Then run the explain analyze again to see if it is faster. If it is faster without merge join, then you could try to change your settings to make the planner prefer the nested loop. I'm not sure what the best way to do that is. Maybe you could try reducing the random_page_cost, which should make index scans cheaper. Dave ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [PERFORM] Big diference in response time (query plan question)
-Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Luiz K. Matsumura Where I can see the current random_page_cost value ? There are some hint about what value I must set ? Thanks in advance. Luiz On Linux the random_page_cost is set in the postgresql.conf file. You can see what it is set to by typing show random_page_cost. This page has some guidelines on random_page_cost and other server settings: http://www.powerpostgresql.com/PerfList/ As it says on the page, make sure you test a variety of queries. ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
[PERFORM] Big diference in response time (query plan question)
Hi all, I have PostgreSQL 8.1.4 running on a P 4 2.8 GHz , 512 MB with Linux (Fedora Core 3) The SQL comands below have a performance diference that I think is not so much acceptable ( 1035.427 ms vs 7.209 ms ), since the tables isn´t so much big ( contrato have 1907 rows and prog have 40.002 rows ) Can I make some optimization here ? EXPLAIN ANALYZE SELECT Contrato.Id , Min( prog.dtsemeio ) AS DtSemIni , Max( prog.dtsemeio ) AS DtSemFim , Min( prog.dtembarque ) AS DtEmbIni , Max( prog.dtembarque ) AS DtEmbFim , Min( prog.dtentrega ) AS DtEntIni , Max( prog.dtentrega ) AS DtEntFim , COUNT(prog.*) AS QtSem , SUM( CASE WHEN Prog.DtSemeio = '20060814' THEN 1 ELSE 0 END ) AS QtSemAb FROM bvz.Contrato LEFT OUTER JOIN bvz.Prog ON prog.Fk_Contrato = Contrato.Id WHEREContrato.Fk_Clifor = 243 GROUP BY 1; QUERY PLAN GroupAggregate (cost=5477.34..5706.84 rows=41 width=48) (actual time=883.721..1031.159 rows=41 loops=1) - Merge Left Join (cost=5477.34..5686.15 rows=860 width=48) (actual time=868.038..1026.988 rows=1366 loops=1) Merge Cond: (outer.id = inner.fk_contrato) - Sort (cost=50.39..50.49 rows=41 width=4) (actual time=0.614..0.683 rows=41 loops=1) Sort Key: contrato.id - Bitmap Heap Scan on contrato (cost=2.14..49.29 rows=41 width=4) (actual time=0.163..0.508 rows=41 loops=1) Recheck Cond: (fk_clifor = 243) - Bitmap Index Scan on fki_contrato_clifor (cost=0.00..2.14 rows=41 width=0) (actual time=0.146..0.146 rows=41 loops=1) Index Cond: (fk_clifor = 243) - Sort (cost=5426.95..5526.95 rows=40002 width=48) (actual time=862.192..956.903 rows=38914 loops=1) Sort Key: prog.fk_contrato - Seq Scan on prog (cost=0.00..1548.02 rows=40002 width=48) (actual time=0.044..169.795 rows=40002 loops=1) Total runtime: 1035.427 ms EXPLAIN ANALYZE SELECT Contrato.Id , Min( prog.dtsemeio ) AS DtSemIni , Max( prog.dtsemeio ) AS DtSemFim , Min( prog.dtembarque ) AS DtEmbIni , Max( prog.dtembarque ) AS DtEmbFim , Min( prog.dtentrega ) AS DtEntIni , Max( prog.dtentrega ) AS DtEntFim , COUNT(prog.*) AS QtSem , SUM( CASE WHEN Prog.DtSemeio = '20060814' THEN 1 ELSE 0 END ) AS QtSemAb FROM bvz.Contrato LEFT OUTER JOIN bvz.Prog ON prog.Fk_Contrato = Contrato.Id WHEREContrato.Fk_Clifor = 352 GROUP BY 1; QUERY PLAN GroupAggregate (cost=2.16..4588.74 rows=28 width=48) (actual time=2.196..7.027 rows=28 loops=1) - Nested Loop Left Join (cost=2.16..4574.63 rows=587 width=48) (actual time=2.042..6.154 rows=223 loops=1) - Index Scan using pk_contrato on contrato (cost=0.00..100.92 rows=28 width=4) (actual time=1.842..3.045 rows=28 loops=1) Filter: (fk_clifor = 352) - Bitmap Heap Scan on prog (cost=2.16..159.19 rows=47 width=48) (actual time=0.040..0.080 rows=8 loops=28) Recheck Cond: (prog.fk_contrato = outer.id) - Bitmap Index Scan on fki_prog_contrato (cost=0.00..2.16 rows=47 width=0) (actual time=0.018..0.018 rows=8 loops=28) Index Cond: (prog.fk_contrato = outer.id) Total runtime: 7.209 ms I think that the problem is in LEFT OUTER JOIN because when I run the queries with a inner join I have more consistent times, although the query plan above is a champion : EXPLAIN ANALYZE SELECT Contrato.Id , Min( prog.dtsemeio ) AS DtSemIni , Max( prog.dtsemeio ) AS DtSemFim , Min( prog.dtembarque ) AS DtEmbIni , Max( prog.dtembarque ) AS DtEmbFim , Min( prog.dtentrega ) AS DtEntIni , Max( prog.dtentrega ) AS DtEntFim , COUNT(prog.*) AS QtSem , SUM( CASE WHEN Prog.DtSemeio = '20060814' THEN 1 ELSE 0 END ) AS QtSemAb FROM bvz.Contrato JOIN bvz.Prog ON prog.Fk_Contrato = Contrato.Id WHEREContrato.Fk_Clifor = 243 GROUP BY 1; QUERY PLAN HashAggregate (cost=1825.38..1826.71 rows=41 width=48) (actual time=222.671..222.788 rows=41 loops=1) - Hash Join (cost=49.40..1806.03 rows=860 width=48) (actual time=2.040..217.963 rows=1366 loops=1) Hash Cond: (outer.fk_contrato = inner.id) - Seq Scan on prog (cost=0.00..1548.02 rows=40002 width=48) (actual time=0.047..150.636
