Posted by sqlservernewbie on 04/13/07 01:23

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.

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