[Bizgres-general] Re: Mapping from bitmap array position to heap location (was : A bitmap index access method is available...)

[Jie Zhang]

On 1/3/06 2:43 AM, "Simon Riggs" <simon at 2ndquadrant.com> wrote:

> On Tue, 2006-01-03 at 11:05 +1300, Mark Kirkwood wrote:
>> Jie Zhang wrote:
> 
>>> One interesting point is that you choose 2048 as max offset number. Is this
>>> true for all cases? If so, we can set the word size to 8-bit to further
>>> increase the bitmap compression rate.
> 
>> If sizeof(ItemIdData) is always *at least* 4 bytes (which seems
>> reasonable) then the biggest MaxOffsetNumber can be is 32768/4 = 8192
>> (i.e if you choose 32K pages).
> 
> We're always pointing to heap blocks, for which the fixed overhead is
> always at least another 28 bytes. So the actual maximum with 32K pages
> is about 1000, even assuming the table is 1 column, nullable, with null
> value. 
> 

Thank you both for these explanations. I will test if I can take advantage
of this.

Best,
Jie