[Bizgres-general] A bitmap index access method is available in bizgres CVS tip

[Jim C. Nasby]

On Tue, Dec 20, 2005 at 10:31:37PM -0800, Jie Zhang wrote:
> There are total 10,000,000 uncompressed bits for each value. Consider the
> word size is 32, then there are 10,000,000/32 = 312,500 words. Since the
> values are evenly distributed in the table, then every 4 words(120 bits)
> will be compressed into about 2 words, because bit 1 will happen in every
> 100 bits. So there are 312,500/2 = 156,250 words in a compressed bit vector.
> Currently, each bitmap page can store about 225 words. Therefore, the number
> of bitmap pages for a value can be 156,250/225=695. That is about
> 695*8192=5.693M. So for 100 distinct values, the bitmap index will take up
> about 569M, which matches what you get there, and this should be the worse
> case.

I'm probably missing something obvious here, but why would a 100 value
index be 100 times bigger than a 10 value index, instead of just 10x?
-- 
Jim C. Nasby, Sr. Engineering Consultant      jnasby at pervasive.com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461