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Posted by Lemming on 04/28/07 00:21
On 12 Apr 2007 18:23:07 -0700, sqlservernewbie@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.
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