Re: About Oracle Bitmap Index...

From: Richard Foote <richard.foote_at_bigpond.com>
Date: Tue, 13 May 2003 21:55:37 +1000
Message-ID: <LO4wa.33661$1s1.490622@newsfeeds.bigpond.com>

"Stanley" <chiujen.lin_at_msa.hinet.net> wrote in message news:ebf3513d.0305121959.3bc5d527_at_posting.google.com...
> I create many Bitmap Indices on all attributes of a table which has
> about 100 attributes(A1~A100). I query data and use hint on it, when
> the Where clause including some of attributes of A1 to A50, it is good
> to work, but when the Where clause including some of attributes of A51
> to A100, it's performance will
> be very poor, why ???
>

Hi Stanley,

Remember that a bitmap index is just an index, albeit one that (should) use less storage that it's b-tree cousin and one that can be very efficiently combined with other bitmap indexes to determine a set of required rowids.

However, if the resultant number of matching rowids is large enough such that the number of LIOs and associated costs is greater than could be achieved by performing a full table scan, then they're not going to be of much help.

Bitmap indexes are not some magical device that enables us to retrieve large volumes of data quickly. My guess with your scenario is that you're attempting to perform magic with them ;)

Cheers

Richard Received on Tue May 13 2003 - 06:55:37 CDT