So has anyone else applied a covering index in MySQL and gotten improved performance but when you run explain, it shows using where, using temporary under the extra column? Any thoughts why this is the case? I'm happy about the performance improvement but the engineer in me wants to know why the extra column is not saying "using index". If you need me to provide details, I will but this question is directed to the experts who have already experienced this with MySQL and know why. I would add that in my particular case, I applied a covering index to an update statement. All the columns in the query were included in the composite (covering) index. Here is an example of exactly what I did:
Table A has the following properties:
Has the following columns: one, two, three, four, five, six, seven, eight, and nine.
My covering index query target:
update table A set one = 'somevalue', two = 'somevalue2', four = 'somevalue3', five = 'somevalue4' where seven = 'somevalue5' and eight = 'somevalue6'
covering index on table A: idx_covering(seven,eight, one,two,four,five)