If a query or a command (such as count) is doing a table scan, we should note that in the logs explicitly.

In addition, we should add a table scans counter for both queries and commands to the server status results. That way, we can graphs table scans in MMS. This would help us diagnose these kinds of problems without having to look at the logs.

If it's difficult to determine exactly what qualifies as a table scan, I'd recommend a simple heuristic. For instance, we could log whenever nscanned > 10n and when nscanned > 1000.