Re: multi-part index and when it is used?

[Harrison Fisk]

Hi,

"SELECT * FROM phonebook ORDER BY lastName, firstName, middleInitial" normally wouldn't use the index. The only time it would is if the * represents only the columns lastName, firstName, middleInitial (ie. there are only 3 columns in the entire table). I discussed that logic at http://forums.mysql.com/read.php?24,10738,10785#msg-10785

A covering index is when all columns in the query are part of the same index. Then MySQL won't have to read the datafile at all. Since you are retreiving all of the rows above, it won't use the index to do it unless there is a covering index.

Harrison Fisk, Trainer and Consultant
MySQL AB, www.mysql.com