We only allow covering for predicates that can be answered exactly via an index scan. The "tightness" of the bounds is called EXACT in this case. If the bounds are not EXACT, then in order to answer the predicate we must fetch the full document and pass it through the filter.
The 2.6 and 3.0 query engine does not consider range predicates against BinData values EXACT, which means that such queries cannot be covered. However, the 2.4 query engine would allow such predicates to be answered without fetching the document. We should restore the 2.4 behavior.
- links to