[GitHub] willholley commented on issue #863: Fix exists false when field is explicitly defined

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willholley commented on issue #863: Fix exists false when field is explicitly defined
URL: https://github.com/apache/couchdb/pull/863#issuecomment-334692468
 
 
   @tonysun83 if a user specifies `{"$exists": false}` explicitly, we can never use an index to fulfil that predicate (you're correct that this was addressed in https://github.com/apache/couchdb/commit/503f3e34afa0a68e91258ca864e70ab18e6584e2). Can you provide an example where you think we'll trip up (or ideally, a failing test)?
   
   As a general rule, I think we should consider it a bug whenever there is variation in results due to indexes being present, the exception being index-specific operators such as `$text`. The expectation is that a user will start with no indexes (so queries will default to _all_docs) and add indexes when necessary to improve performance or when sorting is required. Adding an index to an existing application should never change the results of queries already being used.
   
   Another thing to note, for awareness, is that an explicit [partial index](https://github.com/apache/couchdb/pull/808) could be used to optimise an `{"$exists": false}` selector but these require users to opt-in - the query planner will never automatically select them.
 
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