Mongo 3.4 是否为此查询使用索引？

Question

我正在使用分片副本集。拥有一个具有以下索引的大型集合：

{ 
    "shop" : 1, 
    "email" : 1
}

我正在尝试找到逐一查询每个元素的最佳方法：

db.user.explain('executionStats').find({shop:151, email:{$exists:true}}, {limit:1}))

结果如下。

我的问题是：查询是否使用索引？如果是，我如何阅读这个 explain 结果？

第二：最好使用这个查询，循环游标，然后再查询

.find({shop:151, email:{$exists:true}}, {limit:100}))

或

.findOne({shop:151, email:{$exists:true}}))

在连续循环中？

{ 
"queryPlanner" : {
    "plannerVersion" : NumberInt(1), 
    "namespace" : "vt4users.user", 
    "indexFilterSet" : false, 
    "parsedQuery" : {
        "$and" : [
            {
                "shop" : {
                    "$eq" : NumberInt(151)
                }
            }, 
            {
                "email" : {
                    "$exists" : true
                }
            }
        ]
    }, 
    "winningPlan" : {
        "stage" : "PROJECTION", 
        "transformBy" : {
            "limit" : NumberInt(1)
        }, 
        "inputStage" : {
            "stage" : "FETCH", 
            "filter" : {
                "email" : {
                    "$exists" : true
                }
            }, 
            "inputStage" : {
                "stage" : "IXSCAN", 
                "keyPattern" : {
                    "shop" : NumberInt(1), 
                    "email" : NumberInt(1)
                }, 
                "indexName" : "shop_1_email_1", 
                "isMultiKey" : false, 
                "isUnique" : false, 
                "isSparse" : true, 
                "isPartial" : false, 
                "indexVersion" : NumberInt(1), 
                "direction" : "forward", 
                "indexBounds" : {
                    "shop" : [
                        "[151.0, 151.0]"
                    ], 
                    "email" : [
                        "[MinKey, MaxKey]"
                    ]
                }
            }
        }
    }, 
    "rejectedPlans" : [

    ]
}, 
"executionStats" : {
    "executionSuccess" : true, 
    "nReturned" : NumberInt(3), 
    "executionTimeMillis" : NumberInt(0), 
    "totalKeysExamined" : NumberInt(3), 
    "totalDocsExamined" : NumberInt(3), 
    "executionStages" : {
        "stage" : "PROJECTION", 
        "nReturned" : NumberInt(3), 
        "executionTimeMillisEstimate" : NumberInt(0), 
        "works" : NumberInt(4), 
        "advanced" : NumberInt(3), 
        "needTime" : NumberInt(0), 
        "needYield" : NumberInt(0), 
        "saveState" : NumberInt(0), 
        "restoreState" : NumberInt(0), 
        "isEOF" : NumberInt(1), 
        "invalidates" : NumberInt(0), 
        "transformBy" : {
            "limit" : NumberInt(1)
        }, 
        "inputStage" : {
            "stage" : "FETCH", 
            "filter" : {
                "email" : {
                    "$exists" : true
                }
            }, 
            "nReturned" : NumberInt(3), 
            "executionTimeMillisEstimate" : NumberInt(0), 
            "works" : NumberInt(4), 
            "advanced" : NumberInt(3), 
            "needTime" : NumberInt(0), 
            "needYield" : NumberInt(0), 
            "saveState" : NumberInt(0), 
            "restoreState" : NumberInt(0), 
            "isEOF" : NumberInt(1), 
            "invalidates" : NumberInt(0), 
            "docsExamined" : NumberInt(3), 
            "alreadyHasObj" : NumberInt(0), 
            "inputStage" : {
                "stage" : "IXSCAN", 
                "nReturned" : NumberInt(3), 
                "executionTimeMillisEstimate" : NumberInt(0), 
                "works" : NumberInt(4), 
                "advanced" : NumberInt(3), 
                "needTime" : NumberInt(0), 
                "needYield" : NumberInt(0), 
                "saveState" : NumberInt(0), 
                "restoreState" : NumberInt(0), 
                "isEOF" : NumberInt(1), 
                "invalidates" : NumberInt(0), 
                "keyPattern" : {
                    "shop" : NumberInt(1), 
                    "email" : NumberInt(1)
                }, 
                "indexName" : "shop_1_email_1", 
                "isMultiKey" : false, 
                "isUnique" : false, 
                "isSparse" : true, 
                "isPartial" : false, 
                "indexVersion" : NumberInt(1), 
                "direction" : "forward", 
                "indexBounds" : {
                    "shop" : [
                        "[151.0, 151.0]"
                    ], 
                    "email" : [
                        "[MinKey, MaxKey]"
                    ]
                }, 
                "keysExamined" : NumberInt(3), 
                "dupsTested" : NumberInt(0), 
                "dupsDropped" : NumberInt(0), 
                "seenInvalidated" : NumberInt(0)
            }
        }
    }
}, 
"serverInfo" : {
    "host" : "host", 
    "port" : NumberInt(27017), 
    "version" : "3.2.11", 
    "gitVersion" : "009580ad490190ba33d1c6253ebd8d91808923e4"
}, 
"ok" : NumberInt(1)

}

Answer 1

您的查询正在使用索引。您可以在 inputStage

中看到

"stage" : "IXSCAN"

您可以check it in the docs更好地理解其含义。

关于你的第二个问题，如果你想获取所有结果，使用 findOne 循环并不比正常查询更好（性能方面）。

Mongo 3.4 是否为此查询使用索引？

Is Mongo 3.4 using indexes for this query or not?

mongodb

indices