Subject Re: High selectivity
Author Alex
--- In, Alexandre Benson Smith
<iblist@t...> wrote:
> Hi all,
> Following this thread, I want to make a comment... I remember this
> discussed some time...
> MSSQL has and index type that is named "clustered index".
> You can only have one index of this type for each table, this kind
of index
> is good for low selectivity indexes. This index is a special type, that
> make the table physical ordered, the leaf pages of the index is the
> page instead of a page of pointers to datapages.
> When you select using this index the db start at the first page and
> continuosly next pages to retrieve teh records, since a bunch of
records is
> "reached" in each page (because it is physically ordered) you can do
> selects on low selectivity index in a good way.
> I hope this can bring some ideas...
> see you
> Alexandre Benson Smith
> Development
> THOR Software e Comercial Ltda.
> Santo Andre - Sao Paulo - Brazil
> P.S. I never used but I read sometime that Oracle has Bitmap indexes
to be
> used on low selectivity indexes

sorry you mixed up again some things:

bitmap indexes (yes oracle has them, but only the enterprise
edition!!) are good for HIGH selectivity (e.g. table with 1 000 000
records and you often select a few 100 000 records! low selectivity
means that you search/select only a few records maybe 100 or 200. and
for this LOW selectivity you normally use b-trees or b*-trees (if i
am right firebird knows only b-trees).