If the values of a column are being queried often that column should have an index.
If a set of values are queried as a set an index should be made for that set of columns.
I've added indexes to columns and combinations of columns I noticed were queried against often. It seemed to decrease the average response time by about 10%
If the values of a column are being queried often that column should have an index. If a set of values are queried as a set an index should be made for that set of columns.