Re: Bit Map Indexes

Oracle will use as many indexes as are effective to make the query efficient.

In the past I have seen it use 6 out of 8, ignoring two possible indexes because
the added benefit of using them was small.

Queries may also mix b-tree and bitmap indexes, often along the lines of:

	Select rowids from b-tree
	convert rowids to bitmap
	Operate AND/OR/MINUS between created bitmap and existing bitmap
	Convert bitmap result to rowids

Be warned, I have found a problem where operations involving b-tree and bitmap MINUS go wrong.

BTW: bitmap indexes include null values, so a query like:

        select count(*) from table where bitmapped_col is not null; can be satisfied very quickly through a bitmap index.

Gil Givati <atlsyb1_at_inter.net.il> wrote in article <3490090C.6747_at_inter.net.il>...
> Hi,
> I'm looking into using BitMap indexes in a DW environment.
> can someone please tell me if i have more then one B.M index
 involved in
> a query, will orcle use all of them and go to the data, or choose
 just
> one ?
> what if the query involves bitmap index and an ordinary index (with
 very
> low selectivity) ?
>
> Gil.
>
Received on Tue Dec 16 1997 - 00:00:00 CST