Description

When users run an explain of any command (find, aggregate, update, etc.), the plan cache is not used. Instead, the set of candidate plans are generated and a winner chosen without ever consulting the plan cache as described here: https://docs.mongodb.com/manual/core/query-plans/. Furthermore, no plan cache entry is created for the winning plan. We should document this behavior.

This is a common source of user questions and confusion, because when a cache entry is present it can cause users to observe a discrepancy in the plans selected by explain and the plans selected without explain. SERVER-16895 is an open request to allow users to run explain in a mode which read a plan out of the plan cache, but this improvement is not currently scheduled for development.

Scope of changes

• Clarify stated behavior in:
  • explain dbcommand
  • db.collection.explain()
  • cursor.explain()
  • $explain (deprecated)