wohali commented on issue #1566: Mango operator $in does not utilize index URL: https://github.com/apache/couchdb/issues/1566#issuecomment-414012485 @kgeorgiou First, validate that your index is being used with the `/_db/explain` endpoint. Second, my understanding is that `$in` and `$regex` always require the equivalent of a "full table scan" due to the fact that they are inexact operators. This is an inherent limitation of how Mango is implemented under the covers (same MapReduce as JS views), and at present further no acceleration is possible. You can work around this inherent limitation in certain cases; for instance, if you want to perform an `$in` search on `a` but only on documents with `type: foo`, then you can create a [partial index](http://docs.couchdb.org/en/stable/api/database/find.html#find-partial-indexes) on documents of `type: foo` and the search, which will be in linear time against just that subset of documents. Sorry for the disappointment, and hope the above workaround can help you achieve better performance.
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