MongoDB查询优化：从10s到10ms

【摘要】本文是我前同事付秋雷最近遇到到一个关于MongoDB执行计划选择的问题，非常有意思，在探索源码之后，他将整个问题搞明白并整理分享出来。付秋雷曾是Tair（阿里内部用得非常广泛的KV存储系统）的核心开发成员，目前就职于蘑菇街。

背景

苏先生反馈线上某条查询很慢（10+ seconds），语句相当于：

db.myColl.find({app:"my_app",requestTime:{$gte:1492502247000,$lt:1492588800000}}).sort({_id:-1}).limit(1)

myColl这个collection中的记录内容类似于：

{ "_id" : ObjectId("58fd895359cb8757d493ce60"), "app" : "my_app", "eventId" : 141761066, "requestTime" : NumberLong("1493010771753"), "scene" : "scene01" }
{ "_id" : ObjectId("58fd895359cb8757d493ce52"), "app" : "my_app", "eventId" : 141761052, "requestTime" : NumberLong("1493010771528"), "scene" : "scene02" }
{ "_id" : ObjectId("58fd895359cb8757d493ce36"), "app" : "my_app", "eventId" : 141761024, "requestTime" : NumberLong("1493010771348"), "scene" : "scene03" }
{ "_id" : ObjectId("58fd895359cb8757d493ce31"), "app" : "my_app", "eventId" : 141761019, "requestTime" : NumberLong("1493010771303"), "scene" : "scene01" }
{ "_id" : ObjectId("58fd895359cb8757d493ce2d"), "app" : "my_app", "eventId" : 141761015, "requestTime" : NumberLong("1493010771257"), "scene" : "scene01" }
{ "_id" : ObjectId("58fd895259cb8757d493ce10"), "app" : "my_app", "eventId" : 141760986, "requestTime" : NumberLong("1493010770866"), "scene" : "scene01" }
{ "_id" : ObjectId("58fd895259cb8757d493ce09"), "app" : "my_app", "eventId" : 141760979, "requestTime" : NumberLong("1493010770757"), "scene" : "scene01" }
{ "_id" : ObjectId("58fd895259cb8757d493ce02"), "app" : "my_app", "eventId" : 141760972, "requestTime" : NumberLong("1493010770614"), "scene" : "scene03" }
{ "_id" : ObjectId("58fd895259cb8757d493cdf1"), "app