pgj opened a new pull request, #4710:
URL: https://github.com/apache/couchdb/pull/4710

   It is not always beneficial for the performance if the Mango query planner 
tries to assign an index to the selector.  User-specified indexes may save the 
day, but since they are only hints for the planner, automated overrides can 
still happen.
   
   Introduce the concept of "strict index selection" (h/t @willholley) which 
lets the user to request the exclusive use of a specific index.  When it is not 
possible, give up on planning and return an HTTP 400 response right away.  This 
way the user has the chance to learn about the missing index, request its 
creation and try again later.
   
   The feature comes with a configuration toggle.  By default, the feature is 
disabled to maintain backward compatibility, but the user may ask for this 
behavior via the new `use_index_strict` query parameter for a specific query.  
Note that, similarly, `use_index_strict` could be used to disable strict index 
selection temporarily when it is enabled on the server.
   
   Fixes #4511
   
   ## Checklist for the review
   
   - [x] Code is written and works correctly
   - [x] Changes are covered by tests
   - [x] Any new configurable parameters are documented in 
`rel/overlay/etc/default.ini`
   - [x] Documentation changes were made in the `src/docs` folder
   


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