Question about composite indexes and WHERE + GROUP BY
The index isn't covering since B isn't part of the WHERE clause as a predicate, so the index is not applicable to this query. In other words, the index stores the data sorted by A then by B then by C, but since you're only filtering on A and C, there's no direct way to seek out C without B. You might end up seeing an index scan or table scan get used instead, in this case.