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Posted by hpuxrac on 04/28/07 00:52
On Apr 27, 8:21 pm, Lemming <thiswillbou...@bumblbee.demon.co.uk>
wrote:
> On 12 Apr 2007 18:23:07 -0700, sqlservernew...@yahoo.com wrote:
>
>
>
>
>
> >I found out. It is called "COLUMN CARDINALITY"
>
> >Sorry, no prizes.
>
> >http://www.informatik.uni-trier.de/~ley/db/journals/tods/WhangVT90.html
>
> >(1) obtaining the column cardinality (the number of unique values in a
> >column of a relation) and
> >(2) obtaining the join selectivity (the number of unique values in the
> >join column resulting from an unconditional join divided by the number
> >of unique join column values in the relation to Be joined).
>
> >These two parameters are important statistics that are used in
> >relational query optimization and physical database design.
>
> >http://www.idig.za.net/mysqlindexes/2006/11/09/
>
> >Column cardinality. This is the number of unique values contained in a
> >column. Indexes work best when there is a high cardinality. Put
> >another way, the more unique values there are (fewer duplicates) the
> >better that column will be for indexing. Consider the ID number column
> >of the previous example. Here there are no duplicates, only unique
> >values. This column will be ideal for indexing. On the other end of
> >the scale may be the first names column. Here there will probably be a
> >number of duplicate names (fewer unique values) and a lower
> >cardinality compared to the ID column.
>
> Yes, cardinality is the correct term.
>
> Now, for bonus credits: can anyone tell me the correct term for
> someone who posts a homework question here, gets an answer, and then
> pretends he worked the answer out for himself?
>
> Lemming
> --
> Curiosity *may* have killed Schrodinger's cat
Way to jump all over a thread that died 2 weeks ago.
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