If query covered by single index it avoids Fetch.
If query is not covered by single index and more than one index qualifies it does IntersectScan by using all qualified indexes. In that situation query must fetch the document even though all indexes together has required fields.
Reason: Couchbase indexes are maintained asynchronously, each index might be using different snapshot of time.
By combining different sources query can’t produce results. It must fetch keep the document consistent.