Hi,

Iam reading the book by Helen and I have some questions about INDEX.
In JOINS statement, Helen says the column use on the ON criteria if
has index will be use and will greatly improve the performance. But it
doesn't mention which side of the table's column should be index? Is
one side enough? Is left or right side matter? or better with both column?

How is selectivity and statistic related. From the book I learn that
selectivity that closes to 1 is the better(do we have a treshold here
as to what is acceptable is a selectivity of .5 still good?)

Apinot