Re: Theoretical definition for the number of unique values?
Date: 12 Apr 2007 18:23:07 -0700
Message-ID: <1176427387.896489.134810_at_o5g2000hsb.googlegroups.com>
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. Received on Fri Apr 13 2007 - 03:23:07 CEST