Queries with projections limiting returned fields can result in full collection scans

> db.version() 2.6.0-rc0   > db.foo.ensureIndex({ id1:1 }) WriteResult({ "nInserted" : 1 }) > db.foo.ensureIndex({ id2:1 }) WriteResult({ "nInserted" : 1 }) > for( var i=1; i<=500000; i++ ){db.foo.save({ id1:i, id2:i })} WriteResult({ "nInserted" : 1 }) > db.foo.find({ id1:{ $lte:10 }, id2:1 }).explain() { "cursor" : "BtreeCursor id1_1", "isMultiKey" : false, "n" : 1, "nscannedObjects" : 10, "nscanned" : 10, "nscannedObjectsAllPlans" : 12, "nscannedAllPlans" : 15, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 0, "nChunkSkips" : 0, "millis" : 1, "indexBounds" : { "id1" : [ [ -Infinity, 10 ] ] }, "server" : "Jeffs-MacBook-Air.local:27017", "filterSet" : false } > db.foo.find({ id1:{ $lte:10 }, id2:1 }, { _id:0, id1:1, id2:1 }).explain() { "cursor" : "BasicCursor", "isMultiKey" : false, "n" : 1, "nscannedObjects" : 500000, "nscanned" : 500000, "nscannedObjectsAllPlans" : 500003, "nscannedAllPlans" : 500007, "scanAndOrder" : false, "indexOnly" : false, "nYields" : 3906, "nChunkSkips" : 0, "millis" : 303, "server" : "Jeffs-MacBook-Air.local:27017", "filterSet" : false }