Choose what to index

Choosing what to index depends on the structure of your typical query statements. For example, to be used for SELECT statements, indexes have to match at least the beginning of the WHERE clause. A simple index on column A would not be usable for the SELECT statement "WHERE B = 6 and A = 99"; neither would a composite index on columns A and B. A simple index on just column B would be usable; so would a composite index on columns B and C. Best of all, from the standpoint of that specific query, would be an index on columns B and A. It would not matter (again from the standpoint of this specific query example) whether it also included other columns after those.