Re: When to use bitmap indexes

In principle it works like this

For each and every piece of DML the entire bitmap has to be recalculated. Ergo if you have an OLTP type situation the overhead on nomal transactions is far far too high. For a data warehouse of course you drop the index, load and recreate the index.

HTH Niall
"Jim Davis" <jimdavis_at_iprolink.ch> wrote in message news:3bb00528$1_at_news.swissonline.ch...
> Connor,
>
> As one having been contemplating implementing bitmap indexes for a few
> cases, my curious mind wonders if you would be willing to expound on:
>
> "NB: Bitmaps and changes to the tables do NOT mix at all well"
>
> Thanks in advance,
>
> Jim Davis
>
>
>
> "Connor McDonald" <connor_mcdonald_at_yahoo.com> wrote in message
> news:3BABBFEA.6E8D_at_yahoo.com...
> > Michiel Brunt wrote:
> > >
> > > In our data warehouse we have a large fact table with keys to
dimension
> > > tables.
> > > Something like:
> > >
> > > Product_key Customer_key Time_key Revenue
> > > 1 1 1 1000
> > > 1 1 2 2000
> > > 1 2 2 1000
> > > 1 3 1 1000
> > > 1 3 2 2000
> > > 2 1 1 500
> > > 2 1 2 1000
> > > 2 2 2 2000
> > >
> > > This fact table contains 10 mln records.
> > > There are 10.000 products and 500.000 customers
> > > The question is wether the use of bitmap indexes is useful.
> > >
> > > I heard that the number of distinct values in a column should be less
> than
> > > 10% of the total number of rows in the table.
> > > In that case 10.000 / 10.000.000 is fine and even 500.000 / 10.000.000
> would
> > > work.
> > >
> > > Thanks,
> > > Michiel
> >
> > One of the great things with bitmaps is that they take very little time
> > to create, and very little space - so I'd just bung them on and see how
> > you go...
> >
> > NB: Bitmaps and changes to the tables do NOT mix at all well
> > --
> > ==============================
> > Connor McDonald
> >
> > http://www.oracledba.co.uk
> >
> > "Some days you're the pigeon, some days you're the statue..."
>
>
Received on Tue Sep 25 2001 - 14:41:35 CDT