Question

我的mongodb数据库包含每个设备每分钟的文档。我有一个查询基本上搜索与月份字段匹配的文档。

    db.data.aggregate([
     {
       "$match":{ dId:14,month:6}
     },
     {
      "$project" :{
        dayOfMonth_agg :{$dayOfMonth : "$ts"},
        month : "$month",
        year_agg  : {$year : "$ts"},
        hour_agg  : {$hour : "$ts"},
        a0 :1,
        a1:1,
        d2:1,
        f0:1,
        dId:1
      }
    },
   {
    "$match":{year_agg:2017}
},
     {
      $group :{
        _id : "$dayOfMonth_agg",
        totalEnergy : { $sum: { $multiply: [{$subtract:["$a1","$a0"]},"$f0"] } },
        avg_d0:{$avg:"$a0"},
        avg_d1:{$avg:"$a1"},
        avg_d2:{$avg:"$d2"},
      }
     },
     {
       $sort:{"_id":1}
     }
   ])

我想优化查询性能，因此我将复合索引添加为：

db.data.createIndex({dId:1,month:1})

现在，在执行统计信息中，当创建dId上的单个索引时，docsExamined等于stats的值。在dId上使用单个索引的Exec统计数据：

"keysExamined" : 103251,
"docsExamined" : 103251,
"hasSortStage" : true,
"cursorExhausted" : true,
"numYield" : 814,
"locks" : {
    "Global" : {
        "acquireCount" : {
            "r" : NumberLong(1654)
        }
    },
    "Database" : {
        "acquireCount" : {
            "r" : NumberLong(827)
        }
    },
    "Collection" : {
        "acquireCount" : {
            "r" : NumberLong(826)
        }
    }
},
"nreturned" : 8,
"responseLength" : 770,
"protocol" : "op_command",
"millis" : 656,
"planSummary" : "IXSCAN { dId: 1 }",
"ts" : ISODate("2017-11-15T08:41:50.782Z"),

具有复合指数（dId和月）的执行数据

"keysExamined" : 103251,
"docsExamined" : 103251,
"hasSortStage" : true,
"cursorExhausted" : true,
"numYield" : 810,
"locks" : {
    "Global" : {
        "acquireCount" : {
            "r" : NumberLong(1646)
        }
    },
    "Database" : {
        "acquireCount" : {
            "r" : NumberLong(823)
        }
    },
    "Collection" : {
        "acquireCount" : {
            "r" : NumberLong(822)
        }
    }
},
"nreturned" : 8,
"responseLength" : 770,
"protocol" : "op_command",
"millis" : 678,
"planSummary" : "IXSCAN { dId: 1, month: 1 }",
"ts" : ISODate("2017-11-15T08:45:38.536Z"),
"client" : "127.0.0.1",
"appName" : "MongoDB Shell",
"allUsers" : [ ],
"user" : ""

据我所知，与单一索引相比，docsExamined应该减少了。为什么会这样？

编辑：请参阅第一条评论以获取答案

复合索引不减少docsExamined

0 个答案: