Is an index on A, B redundant if there is an index on A, B, C?

Question

Having years of experience as a DBA, I do believe I know the answer to the question, but I figured it never hurts to check my bases.

Using SQL Server, assuming I have a

Answer 1

Yes, this is a common optimization. Any query that would benefit from the index on A,B can also benefit just as well from the index on A,B,C.

In the MySQL community, there's even a tool to search your whole schema for redundant indexes: http://www.percona.com/doc/percona-toolkit/pt-duplicate-key-checker.html

The possible exception case would be if the index on A,B were more compact and used much more frequently, and you wanted to control which index was kept loaded in memory.