Hi Peter,
Regarding:
"Selectivity is known -- since it's a primary key, which is unique -- it will
be 1. Cardinality can vary."
I wonder if you're referring to the definition of *cardinality* as used in
mathematics
http://en.wikipedia.org/wiki/Cardinality
In mathematics, the cardinality of a set is a measure of the "number of
elements of the set".
as opposed to the definition common in SQL:
http://en.wikipedia.org/wiki/Cardinality_%28SQL_statements%29
"In SQL (Structured Query Language), the term cardinality refers to the
uniqueness of data values contained in a particular column (attribute) of a
database table. "
Regards,
Donald
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