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Home -> Community -> Usenet -> c.d.o.server -> Re: Hint to get this Query plan?
Greg Stark wrote:
>
> "Jonathan Lewis" <jonathan_at_jlcomp.demon.co.uk> writes:
>
> > Given the normal size and speed of
> > creation of bitmaps - if this query is
> > important you could try creating a
> > bitmap on (a,b).
>
> But the "normal size" is dependent on it being a good candidate for bitmaps.
> If the cardinality of column a is low (in this case ~300) and the cardinality
> of b is low (in this case 4) it does not necessarily follow that the
> cardinality of (a,b) is low (~1200). In this case it might still be
> reasonable, but if b were another 300 then it wouldn't make any sense.
>
> --
> greg
This argument is valid but it has also been shown with some benchmarking that bitmap indexes can out perform their b-tree companions even with extremely high cardinality (mainly because of the reduced size).
There is a paper on the Oracle web site (I can't remember what its called) which showed that even a bitmap with 500,000 distinct values (in a 1,000,000 table) had comparable size and performance with a b-tree.
HTH
--
There are 3,434,679 oracle websites around the world. Now there is one more... http://www.oracledba.co.uk Received on Wed Feb 09 2000 - 04:27:00 CST
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