Re: bitmap-index-scan slower than normal index scan
От | Alex Deucher |
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Тема | Re: bitmap-index-scan slower than normal index scan |
Дата | |
Msg-id | a728f9f90707111152w5e6d7316md4df25c33965c5d4@mail.gmail.com обсуждение исходный текст |
Ответ на | bitmap-index-scan slower than normal index scan (Andreas Kretschmer <akretschmer@spamfence.net>) |
Ответы |
Re: bitmap-index-scan slower than normal index scan
("A. Kretschmer" <andreas.kretschmer@schollglas.com>)
|
Список | pgsql-performance |
On 7/11/07, Andreas Kretschmer <akretschmer@spamfence.net> wrote: > Hi, > > Okay, i know, not really a recent version: > PostgreSQL 8.1.4 on i386-pc-linux-gnu, compiled by GCC cc (GCC) 3.3.5 (Debian 1:3.3.5-13) > > I have a fresh ANALYZED table with some indexes. > > scholl=*# set enable_bitmapscan=1; > SET > scholl=*# explain analyse select sum(flaeche) from bde_meldungen where maschine=1200 and ab = 347735; > QUERY PLAN > -------------------------------------------------------------------------------------------------------------------------------------------------- > Aggregate (cost=1371.95..1371.96 rows=1 width=8) (actual time=163.788..163.790 rows=1 loops=1) > -> Bitmap Heap Scan on bde_meldungen (cost=1217.69..1371.85 rows=39 width=8) (actual time=163.702..163.758 rows=2loops=1) > Recheck Cond: ((ab = 347735) AND (maschine = 1200)) > -> BitmapAnd (cost=1217.69..1217.69 rows=39 width=0) (actual time=163.681..163.681 rows=0 loops=1) > -> Bitmap Index Scan on idx_ab (cost=0.00..5.95 rows=558 width=0) (actual time=0.078..0.078 rows=109 loops=1) > Index Cond: (ab = 347735) > -> Bitmap Index Scan on idx_maschine (cost=0.00..1211.49 rows=148997 width=0) (actual time=163.459..163.459rows=164760 loops=1) > Index Cond: (maschine = 1200) > Total runtime: 163.901 ms > (9 rows) > > > Okay, 163.901 ms with Bitmap Index Scan. > > And now i disable this and runs the same select: > > scholl=*# set enable_bitmapscan=0; > SET > scholl=*# explain analyse select sum(flaeche) from bde_meldungen where maschine=1200 and ab = 347735; > QUERY PLAN > -------------------------------------------------------------------------------------------------------------------------------- > Aggregate (cost=2142.77..2142.78 rows=1 width=8) (actual time=0.229..0.231 rows=1 loops=1) > -> Index Scan using idx_ab on bde_meldungen (cost=0.00..2142.67 rows=39 width=8) (actual time=0.046..0.209 rows=2loops=1) > Index Cond: (ab = 347735) > Filter: (maschine = 1200) > Total runtime: 0.326 ms > (5 rows) > > Okay, i got a really different plan, but i expected _NOT_ a > performance-boost like this. I expected the opposite. > > > It's not a really problem, i just played with this. But i'm confused > about this... > your results are getting cached. try two queries in a row with the same plan. Alex
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