Re: bitmap-index-scan slower than normal index scan

On 7/11/07, Andreas Kretschmer <akretschmer(at)spamfence(dot)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=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..163.459 rows=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...
>

your results are getting cached. try two queries in a row with the same plan.

Alex