Re: bitmap indexes - performance

Leonardo F wrote:
> Using as a starting point the old bitmap patch in:
> 
> http://archives.postgresql.org/message-id/20081101000154.GO27872@fune
> 
> 
> I re-applied and re-worked the patch to see what kind of improvements over
> btrees bitmaps actually provided.
> 
> Using a 20M rows table of 10/100/1000 random values, I've found that:
> 
> 1) bulk index creation time is roughly 6 times better
> 2) index size is 6-15 times smaller (depending on column cardinality)
> 3) there's almost no difference in query times (but I have to make more
> tests)
> 4) I can't say anything about the insertion performance, but I guess
> bitmap will perform way worse than btree
> 
> Are these improvements (index creation time, index size) worth enough
> to keep on working on this?
> 
> I mean: given that bitmaps don't give any benefits in query times, but
> only benefits related to disk size and bulk index creation times, and
> will have horrible performance for insertions/deletions: would this job be
> worthed?
> 
> In case it is: I will try to clean up the patch and post it...
> 
> 
> As a side note: I guess that most of the bitmap indexes performance 
> improvements in the SELECT area are already implemented in postgres
> in the bitmapand/or and bitmap scan stuff? I couldn't find any docs that
> say that bitmap indexes are faster for selects, unless of course they
> are ANDed/ORed together (which is something postgres already does
> for regular btree indexes)

Great report, thanks.  The other big problem with on-disk bitmap indexes
is removing expired values via vacuum.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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