One useful feature of other database systems is the query log. It allows the DBA to review the queries run, who's run them, time taken, &c. This serves both as an audit and also as a source of "ground truth" for performance optimization. For instance, which columns should be indexed. It may also serve as the foundation for automated performance recommendations/actions.
What queries are being run is the first piece. Have this data tied into tracing results and perhaps client-side metrics (
PHOENIX-1819) becomes very useful.
This might take the form of clients writing data to a new system table, but other implementation suggestions are welcome.