in mongo shell, Doing this db.listing.find( {'p_key': 'DhpPsG'} ).count() and db.listing.find({'$query': {'p_key': 'DhpPsG'}}).count() has different result. The former gives result of 1313 while the latter gives 0. -------- Same as for $hint and hint(). Doing this db.listing.find({'$min': {'_id': '5301fc8327f4f0578d70b24ff6bcc8dd', 'created_on': ISODate("2012-12-26T05:14:35.834Z")} , '$query': {'p_key': 'DhpPsG'}}).hint( {'p_key': 1} ).sort( [("created_on", 1), ("_id", 1)] ).explain() uses the index as I expected but doing this db.listing.find({'$min': {'_id': '5301fc8327f4f0578d70b24ff6bcc8dd', 'created_on': ISODate("2012-12-26T05:14:35.834Z")} , '$query': {'p_key': 'DhpPsG'} , '$hint':{'p_key': 1}}).sort( [("created_on", 1), ("_id", 1)] ).explain() does not use the index as I expected. The result of former is: { "cursor" : "BtreeCursor p_key_1", "isMultiKey" : false, "n" : 0, "nscannedObjects" : 83260, "nscanned" : 83260, "nscannedObjectsAllPlans" : 83260, "nscannedAllPlans" : 83260, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 379, "indexBounds" : { "p_key" : [ [ { "$minElement" : 1 } , { "$maxElement" : 1 } ] ] }, "server" : "mbp.local:27017" } the result of latter is: { "cursor" : "BasicCursor", "isMultiKey" : false, "n" : 0, "nscannedObjects" : 83260, "nscanned" : 83260, "nscannedObjectsAllPlans" : 83260, "nscannedAllPlans" : 83260, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 1, "nChunkSkips" : 0, "millis" : 620, "indexBounds" : { }, "server" : "mbp.local:27017" }