Re: mysterious difference in speed when combining two queries with OR - Mailing list pgsql-performance
From | Hans Ekbrand |
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Subject | Re: mysterious difference in speed when combining two queries with OR |
Date | |
Msg-id | 20080423092205.GI11886@amin Whole thread Raw |
In response to | Re: mysterious difference in speed when combining two queries with OR ("A. Kretschmer" <andreas.kretschmer@schollglas.com>) |
Responses |
Re: mysterious difference in speed when combining two queries with OR
|
List | pgsql-performance |
On Wed, Apr 23, 2008 at 10:57:04AM +0200, A. Kretschmer wrote: > am Wed, dem 23.04.2008, um 9:58:10 +0200 mailte A. Kretschmer folgendes: > > > Query A (two queries) > > > > > > select distinct moment.mid from moment,timecard where parent = 45 and (pid=17 and timecard.mid = moment.mid) orderby moment.mid; > > > select distinct moment.mid from moment,timecard where parent = 45 and (pbar = 0) order by moment.mid; > > > > > > Query B (combining the two with OR) > > > > > > select distinct moment.mid from moment,timecard where parent = 45 and ((pid=17 and timecard.mid = moment.mid) or (pbar= 0)) order by moment.mid; > > Thanks to depesz on #postgresql (irc-channel): > > Query A, the second query: there are no join between the 2 tables. > Mistake? No, I just wanted to show the time differences, I haven't used join before. Now that you have adviced me to, I have tried your suggestion to rewrite B as a union and it works good! Just as fast as the A Query! QUERY PLAN -------------------------------------------------------------------------------------------------------------------------------------------------------------- Unique (cost=4100.27..4101.39 rows=223 width=4) (actual time=120.963..121.124 rows=42 loops=1) -> Sort (cost=4100.27..4100.83 rows=223 width=4) (actual time=120.959..121.008 rows=43 loops=1) Sort Key: mid -> Append (cost=287.66..4091.57 rows=223 width=4) (actual time=11.274..120.795 rows=43 loops=1) -> Subquery Scan "*SELECT* 1" (cost=287.66..290.70 rows=203 width=4) (actual time=11.270..11.604 rows=41loops=1) -> Unique (cost=287.66..288.67 rows=203 width=4) (actual time=11.264..11.469 rows=41 loops=1) -> Sort (cost=287.66..288.16 rows=203 width=4) (actual time=11.260..11.331 rows=57 loops=1) Sort Key: moment.mid -> Hash Join (cost=60.98..279.88 rows=203 width=4) (actual time=2.563..11.136 rows=57loops=1) Hash Cond: ("outer".mid = "inner".mid) -> Seq Scan on timecard (cost=0.00..211.78 rows=1017 width=4) (actual time=0.032..7.156rows=995 loops=1) Filter: (pid = 17) -> Hash (cost=59.88..59.88 rows=444 width=4) (actual time=2.329..2.329 rows=0 loops=1) -> Seq Scan on moment (cost=0.00..59.88 rows=444 width=4) (actual time=0.035..1.980rows=199 loops=1) Filter: (parent = 45) -> Subquery Scan "*SELECT* 2" (cost=192.62..3800.87 rows=20 width=4) (actual time=0.583..109.073 rows=2loops=1) -> Unique (cost=192.62..3800.67 rows=20 width=4) (actual time=0.578..109.061 rows=2 loops=1) -> Nested Loop (cost=192.62..3417.57 rows=153240 width=4) (actual time=0.576..89.437 rows=15324loops=1) -> Index Scan using moment_mid_idx on moment (cost=0.00..160.15 rows=20 width=4) (actualtime=0.544..3.527 rows=2 loops=1) Filter: ((parent = 45) AND (pbar = 0)) -> Materialize (cost=192.62..269.24 rows=7662 width=0) (actual time=0.014..21.930 rows=7662loops=2) -> Seq Scan on timecard (cost=0.00..192.62 rows=7662 width=0) (actual time=0.005..14.560rows=7662 loops=1) Total runtime: 122.076 ms (23 rows) -- Hans Ekbrand (http://sociologi.cjb.net) <hans@sociologi.cjb.net> A. Because it breaks the logical sequence of discussion Q. Why is top posting bad?
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