For MySQL Server 4 and below, simple indexes and compound indexes with three or less values are ignored. For MySQL Server 5 and above, the heuristic is more vag

For MySQL Server 4 and below, simple indexes and compound indexes with three or less values are ignored. For MySQL Server 5 and above, the heuristic is more vague. Anyhow, if one or more columns has a combined total of three or more values then an index could reduce a table scan to a range scan.