Re: Index question

[Arthur Fuller]

The difference is based on the relative frequency of queries where a is
important vs. where b is important. Either way, what will happen is that the
index scan will isolate the first item mentioned, then scan the result set
to isolate the second term.

e.g.

SELECT * FROM someTable WERE a = someValue and b = someOtherValue

Step one isolates the matching "a" values.
Step two walks through that resultset and examines each value of b.

Since the values of b are already in memory, all it has to do is examine the
index keys to find the matches of b. No additional disk read is required.

Arthur