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Home -> Community -> Usenet -> c.d.o.server -> Re: When to use bitmap index based on cardinality
Martin Haltmayer wrote:
>
> How about updates/inserts/deletes? I was always told that bitmap indexes are
> weak here.
>
> Martin Haltmayer
>
> Christopher M. Day schrieb:
> >
> > Cutler,
> >
> > (A view from 8.0.5.11)
> > I would agree, up until a couple of days ago ! Check out the
> > proceedings from the OpenWorld'98 conference in SF on www.oracle.com.
> > There was a presentation about using bitmaps on everything but UNIQUE
> > columns.
> >
> > I tried this on two tables. One contains a unique key for all the
> > households in the UK (hurn key) (b*tree), its child contains the hurn
> > key and a person unique key. I used to have a b*tree on the child hurn
> > column and have since changed it to a bitmap. The result was less
> > storage and an increase in performance...
> >
> > Chris
> >
> > cutler_temp_at_usa.net wrote:
> > >
> > > I'm on Oracle 7.3.4.2
> > >
> > > The Oracle Tuning documentation says this about bitmap indexes:
> > > <
> > > Bitmap indexes can substantially improve performance of queries with the
> > > following characteristics:
> > >
> > > The WHERE clause contains multiple predicates on low- or medium-cardinality
> > > columns....
My understanding is that when you update a value, all rows in the table that have that value will also be locked....Thus if your table has two distinct values, then updating one row could conceivably lock half the table...
HTH
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