mongodb用explain()，为啥看不到执行时间，和文档扫描数量？

db.playlists.find({}).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "XXXXX.playlists", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ ] }, "winningPlan" : { "stage" : "COLLSCAN", "filter" : { "$and" : [ ] }, "direction" : "forward" }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "XXXXXXXX", "port" : XXXXX, "version" : "3.0.6", "gitVersion" : "1ef45a23a4c5e3480ac919b28afcba3c615488f2" }, "ok" : 1 }