Ability to specify index for query build from

Hi,
after migrating some tables to InnoDB I notice very slow queries when using COUNT().
The solution is to force an index to the DB, something that is not possible to do with CI currently (or maybe I haven't found a way). For the moment I am using hardcoded SQL queries, but this is not the way to go I think.

Am I missing something or do I need to hack the DB_query_builder class?

/Mattias