Sparse indexes normally can't answer inequality queries because they don't know all the unindexed documents, but the exception to that is inequality to null since it's just a subset of $exists:true predicate which sparse indexes can answer.
This ticket is to allow sparse indexes to be used for {$ne:null} queries.
Additionally, we will add $** testing to null_query_semantics.js
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SERVER-36681 Change {$ne: null} semantics to be more intuitive
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- Backlog
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SERVER-19742 Equality to null predicates can sometimes be answered using a sparse, compound index
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- Backlog
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