Since version 3.0, the Query Planner has monitored the performance of cached plans, and replanned operations using them if the cached plan is found to be less efficient than expected. The operation is then logged with the replanned field set to *true.

Since SERVER-40805, those log entries have also included a second field replanReason, which (nearly) always is "cached plan was less efficient than expected: expected trial execution to take X works but it took at least 10X works"

Slow Query diagnosis and resolution would be greatly helped if more detail was logged.

Of most use would be if a new field replanSummary was added, holding what would have been in planSummary if the operation had not been replanned. The summaries would be:

• replanSummary: old plan that was discarded
• planSummary: new plan that replaced it.

I'm hoping that this is just a matter of copying the current contents of the field that ends up being logged as planSummary before it is overwritten by the replanning process ... but I suspect it will not be that simple.