Right now we have some older code for characterizing index suitability for standard btree indexes in the query optimizer, and some newer code in IndexSpec/IndexType implementations for btree and geo indexes. For btree indexes the IndexSpec code tends to be a quick estimate of the suitability while the query optimizer itself provides a definitive determination of suitability. It might be nice to unify these implementations under the IndesSpec/IndexType interface (or some other index interface), for clarity, and so there is less special purpose code for specific index types in the query optimizer.