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

[Jie Zhang]

On 12/19/05 12:25 AM, "Mark Kirkwood" <markir at paradise.net.nz> wrote:

> Mark Kirkwood wrote:
> 
> One other thing comes to mind - apologies for the drip-feed of the
> thoughts on this, but its how they are coming to me as well :-)
> 
> In fact *all* the (bitmapped) indexes seem to have a fairly large
> on-disk footprint - about the same as btree. Is that expected?
> 
> I have not looked specifically at how this has been implemented (am
> going to tomorrow), But thinking aloud (so to speak), concerning my
> (possibly completely wrong - apologies if so) theoretical understanding
> of how bitmap indexes work:
> 
> 10 value bitmap for 10000000 rows requires 10*10000000/8 = 12500000 (~12Mb).
> 
> This is quite small (whereas the 10 value bitmap indexes are ~150Mb).
> 
> To actually use the bitmap you need a map between the bitmap "array"
> position and blocknumber,itemid (or similar) of the heap relation. I am
> guessing that is what your "bm_internal_t*" guys do(?).  Is this where
> the extra space is being used? (the on-disk bm_* are very small, so I
> guess it is not).

Those "bm_internal_t*" are used to store distinct values for each attribute
to be indexed. They are used to speed up the search performance when the
number of distinct values becomes large.