bitmap-index-scan slower than normal index scan - Mailing list pgsql-performance

From Andreas Kretschmer
Subject bitmap-index-scan slower than normal index scan
Date
Msg-id 20070711183638.GA30872@KanotixBox
Whole thread Raw
Responses 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>)
List 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)
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