bitmap-index-scan slower than normal index scan
От | Andreas Kretschmer |
---|---|
Тема | bitmap-index-scan slower than normal index scan |
Дата | |
Msg-id | 20070711183638.GA30872@KanotixBox обсуждение исходный текст |
Ответы |
Re: bitmap-index-scan slower than normal index scan
("Alex Deucher" <alexdeucher@gmail.com>)
Re: bitmap-index-scan slower than normal index scan (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-performance |
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=2 loops=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=2 loops=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... Andreas -- Really, I'm not out to destroy Microsoft. That will just be a completely unintentional side effect. (Linus Torvalds) "If I was god, I would recompile penguin with --enable-fly." (unknow) Kaufbach, Saxony, Germany, Europe. N 51.05082°, E 13.56889°
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