Things exit the cache for:
1. safety reasons (index add/drop)
2. no more space in cache
3. performance degrades
4. writes (changes data distribution)
5. explicit commands to mutate cache

Let's log when this happens w/level...1.

The query optimizer currently generates a query plan which is cached and re-used based on the normalized "shape" of the query predicate and sort criteria.

It would be useful to be able to identify plans by id (e.g., hash value) for tracking, hinting and logging.

1. Tracking
   Store the history when the query plan is changed and what it was changed to in the db's system.plans collection.
2. Diagnostics
   Log every time the plan changes for a given query, e.g., <query_id>: changed <old_plan_id> to <new_plan_id>