在capped-collection上按反向插入顺序排序的最快方法是什么('rf'已被稀疏索引)db.log.find({rf:'o-5556457634'}).sort({'$natural':-1}).explain();{"cursor":"ReverseCappedCursor","nscanned":1654468,"nscannedObjects":1654468,"n":4,"millis":2932,"nYields":5,"nChunkSkips":0,"isMultiKey":false,"indexOnly":false,"indexBounds":{}}似乎“自