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

[Mark Kirkwood]

Jie Zhang wrote:
> 
> On 12/18/05 10:12 PM, "Mark Kirkwood" <markir at paradise.net.nz> wrote:

>>That (bitmapped) bitmaptest_val2 is relatively big at ~600Mb, so I'm
>>wondering if the 100 distinct values is just at the point where the
>>bitmap is getting inflated. Thoughts?
>>
> 
> 
> For each distinct value of val2, the number of bitmap pages can be roughly
> calculated as follows.
> 
> 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.
>
>

Jie,

Thanks for explaining - I'm part way through reading the code, and will 
spend the next few days (trying) to get to grips with it!

Cheers

Mark