Mongodb查询分析器解析

我会带着你远行 2022-09-06 00:18 226阅读 0赞

#### Mongodb查询分析器 ####

*  动态相关项目中涉及到数据量大和吞吐量的接口，例如关注页面动态，附近动态，这部分数据都是存储在mongodb中，在线上数据中分类两个mongodb集合存储
 *  其中关注动态基于扩散写的设计，数据量已经快到 8 亿。
 *  ![在这里插入图片描述][9ca839a5dc794b7b9dae5abf49c8ae1b.png]
 *  附近动态并没有写扩散的数据扩大，并且曝光量相比关注页面少一点，并且重要的是有TTL，因此附近动态相关的表数据50万以上  
    ![在这里插入图片描述][e5a62ee3b1a743da841fbf8148a9fda2.png]  
    ![在这里插入图片描述][b1d24971ce284f249be8716b80af3811.png]
 *  在大数据量查询下，优化查询速度是提高系统性能的一个关键点，在Mysql中我们有explan 诊断工具，来帮助我们来对sql进行分析，同样的Mongodb中获取查询相关信息的诊断工具之一就是explain() 函数，在《MongoDB权威指南》（我只看了索引这一章）书中解释：

explain会返回查询使用的索引情况(如果有的话)，耗时及扫描文档数的统计信息。.....使用explain就会得到当前查询所使用的索引，消耗了多少时间，已经数据库需要扫描多少文档才能得到结果。

*  但是书中使用的Mongodb应该是3.2 版本之前的，因为给出的索引分析案例是如下：

{ 
       "cursor" : "BtreeCursor gender_1_user_name_1",
       "isMultiKey" : false,
       "n" : 1,
       "nscannedObjects" : 0,
       "nscanned" : 1,
       "nscannedObjectsAllPlans" : 0,
       "nscannedAllPlans" : 1,
       "scanAndOrder" : false,
       "indexOnly" : true,
       "nYields" : 0,
       "nChunkSkips" : 0,
       "millis" : 0,
       "indexBounds" : { 
          "gender" : [
             [
                "M",
                "M"
             ]
          ],
          "user_name" : [
             [
                { 
                   "$minElement" : 1
                },
                { 
                   "$maxElement" : 1
                }
             ]
          ]
       }
    }

*  我们在自己数据库中得到完全不一样的输出：

//db.moment.explain(executionStats).find({"subscriberID" : NumberLong(1884831336), "type" : { "$in" : [1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] } }).sort({"auditTime" : -1});
    { 
        "queryPlanner": { 
            "mongosPlannerVersion": 1,
            "winningPlan": { 
                "stage": "SINGLE_SHARD",
                "shards": [{ 
                    "shardName": "cmgo-hgz5ixhj_0",
                    "connectionString": "cmgo-hgz5ixhj_0/100.120.151.75:7004,100.120.152.197:7000,100.93.47.81:7016",
                    "serverInfo": { 
                        "host": "TENCENT64.site",
                        "port": 7016,
                        "version": "3.2.10-cmongo-",
                        "gitVersion": "nogitversion"
                    },
                    "plannerVersion": 1,
                    "namespace": "moment.moment",
                    "indexFilterSet": false,
                    "parsedQuery": { 
                        "$and": [{ 
                            "subscriberID": { 
                                "$eq": NumberLong(1884831336)
                            }
                        },
                        { 
                            "type": { 
                                "$in": [
                                1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
                            }
                        }]
                    },
                    "winningPlan": { 
                        "stage": "SORT",
                        "sortPattern": { 
                            "auditTime": -1
                        },
                        "inputStage": { 
                            "stage": "SORT_KEY_GENERATOR",
                            "inputStage": { 
                                "stage": "FETCH",
                                "filter": { 
                                    "type": { 
                                        "$in": [
                                        1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
                                    }
                                },
                                "inputStage": { 
                                    "stage": "IXSCAN",
                                    "keyPattern": { 
                                        "subscriberID": 1,
                                        "momentID": 1
                                    },
                                    "indexName": "idx_objectid_momentid",
                                    "isMultiKey": false,
                                    "isUnique": true,
                                    "isSparse": false,
                                    "isPartial": false,
                                    "indexVersion": 1,
                                    "direction": "forward",
                                    "indexBounds": { 
                                        "subscriberID": ["[1884831336, 1884831336]"],
                                        "momentID": ["[MinKey, MaxKey]"]
                                    }
                                }
                            }
                        }
                    },
                    "rejectedPlans": [{ 
                        "stage": "FETCH",
                        "inputStage": { 
                            "stage": "SORT_MERGE",
                            "sortPattern": { 
                                "auditTime": -1
                            },
                            "inputStages": [{ 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[19.0, 19.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[18.0, 18.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[17.0, 17.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[16.0, 16.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[15.0, 15.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[14.0, 14.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[13.0, 13.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[12.0, 12.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[11.0, 11.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[10.0, 10.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[8.0, 8.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[7.0, 7.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[6.0, 6.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[5.0, 5.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[4.0, 4.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[3.0, 3.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[2.0, 2.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            },
                            { 
                                "stage": "IXSCAN",
                                "keyPattern": { 
                                    "subscriberID": 1,
                                    "type": 1,
                                    "auditTime": 1
                                },
                                "indexName": "subscriberID_1_type_1_auditTime_1",
                                "isMultiKey": false,
                                "isUnique": false,
                                "isSparse": false,
                                "isPartial": false,
                                "indexVersion": 1,
                                "direction": "backward",
                                "indexBounds": { 
                                    "subscriberID": ["[1884831336, 1884831336]"],
                                    "type": ["[1.0, 1.0]"],
                                    "auditTime": ["[MaxKey, MinKey]"]
                                }
                            }]
                        }
                    },
                    { 
                        "stage": "SORT",
                        "sortPattern": { 
                            "auditTime": -1
                        },
                        "inputStage": { 
                            "stage": "SORT_KEY_GENERATOR",
                            "inputStage": { 
                                "stage": "FETCH",
                                "inputStage": { 
                                    "stage": "IXSCAN",
                                    "keyPattern": { 
                                        "subscriberID": 1,
                                        "unread": 1,
                                        "type": 1
                                    },
                                    "indexName": "subscriberID_1_unread_1_type_1",
                                    "isMultiKey": false,
                                    "isUnique": false,
                                    "isSparse": false,
                                    "isPartial": false,
                                    "indexVersion": 1,
                                    "direction": "forward",
                                    "indexBounds": { 
                                        "subscriberID": ["[1884831336, 1884831336]"],
                                        "unread": ["[MinKey, MaxKey]"],
                                        "type": ["[1.0, 1.0]", "[2.0, 2.0]", "[3.0, 3.0]", "[4.0, 4.0]", "[5.0, 5.0]", "[6.0, 6.0]", "[7.0, 7.0]", "[8.0, 8.0]", "[10.0, 10.0]", "[11.0, 11.0]", "[12.0, 12.0]", "[13.0, 13.0]", "[14.0, 14.0]", "[15.0, 15.0]", "[16.0, 16.0]", "[17.0, 17.0]", "[18.0, 18.0]", "[19.0, 19.0]"]
                                    }
                                }
                            }
                        }
                    }]
                }]
            }
        },
        "executionStats": { 
            "nReturned": 71,
            "executionTimeMillis": 4,
            "totalKeysExamined": 71,
            "totalDocsExamined": 71,
            "executionStages": { 
                "stage": "SINGLE_SHARD",
                "nReturned": 71,
                "executionTimeMillis": 4,
                "totalKeysExamined": 71,
                "totalDocsExamined": 71,
                "totalChildMillis": NumberLong(2),
                "shards": [{ 
                    "shardName": "cmgo-hgz5ixhj_0",
                    "executionSuccess": true,
                    "executionStages": { 
                        "stage": "SORT",
                        "nReturned": 71,
                        "executionTimeMillisEstimate": 0,
                        "works": 146,
                        "advanced": 71,
                        "needTime": 73,
                        "needYield": 0,
                        "saveState": 3,
                        "restoreState": 3,
                        "isEOF": 1,
                        "invalidates": 0,
                        "sortPattern": { 
                            "auditTime": -1
                        },
                        "memUsage": 13682,
                        "memLimit": 33554432,
                        "inputStage": { 
                            "stage": "SORT_KEY_GENERATOR",
                            "nReturned": 0,
                            "executionTimeMillisEstimate": 0,
                            "works": 73,
                            "advanced": 0,
                            "needTime": 1,
                            "needYield": 0,
                            "saveState": 3,
                            "restoreState": 3,
                            "isEOF": 1,
                            "invalidates": 0,
                            "inputStage": { 
                                "stage": "FETCH",
                                "filter": { 
                                    "type": { 
                                        "$in": [
                                        1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
                                    }
                                },
                                "nReturned": 71,
                                "executionTimeMillisEstimate": 0,
                                "works": 72,
                                "advanced": 71,
                                "needTime": 0,
                                "needYield": 0,
                                "saveState": 3,
                                "restoreState": 3,
                                "isEOF": 1,
                                "invalidates": 0,
                                "docsExamined": 71,
                                "alreadyHasObj": 0,
                                "inputStage": { 
                                    "stage": "IXSCAN",
                                    "nReturned": 71,
                                    "executionTimeMillisEstimate": 0,
                                    "works": 72,
                                    "advanced": 71,
                                    "needTime": 0,
                                    "needYield": 0,
                                    "saveState": 3,
                                    "restoreState": 3,
                                    "isEOF": 1,
                                    "invalidates": 0,
                                    "keyPattern": { 
                                        "subscriberID": 1,
                                        "momentID": 1
                                    },
                                    "indexName": "idx_objectid_momentid",
                                    "isMultiKey": false,
                                    "isUnique": true,
                                    "isSparse": false,
                                    "isPartial": false,
                                    "indexVersion": 1,
                                    "direction": "forward",
                                    "indexBounds": { 
                                        "subscriberID": ["[1884831336, 1884831336]"],
                                        "momentID": ["[MinKey, MaxKey]"]
                                    },
                                    "keysExamined": 71,
                                    "dupsTested": 0,
                                    "dupsDropped": 0,
                                    "seenInvalidated": 0
                                }
                            }
                        }
                    }
                }]
            }
        },
        "ok": 1
    }

*  我看到我们测试库的版本是：3.4.21，既然和书上不一样，去官网看看吧，反正书上也是官网的翻译，[Mongodb中explain()函数的网址][Mongodb_explain]，[中文的Mongodb中explain() 网址][Mongodb_explain_]  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70]
 *  如上官网中发现，在3.2版本后新支持了一部分函数的分析，并且在预发规则上也有一点变化

//之前
    db.collection.<method(...)>.explain()
    
    //现在
    db.collection.explain().<method(...)>

*  并且在explain(）函数中还支持参数选择各种模式的分析方式，如下：  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 1]
 *  分为三种模式
    
     *  queryPlanner（默认模式）
     *  executionStats：返回关于查询过程中统计信息，包括使用的随意，扫描的文档数已经查查处理所用时间（毫秒计算）
     *  allPlansExecution：查看计划选择期间收集的部分执行统计信息。
 *  我们主要用的是excutionStats模式来进行分析，如上最长的那个explain返回的json为例：
 *  对queryPlanner分析
    
     *  queryPlanner : 标识queryPlanner的返回
     *  queryPlanner.namespace:该值返回的是该query所查询的表
     *  queryPlanner.indexFilterSet:针对该query是否有indexfilter
     *  queryPlanner.winningPlan:查询优化器针对该query所返回的最优执行计划的详细内容。
     *  queryPlanner.winningPlan.stage:最优执行计划的stage，这里返回是SORT，可以理解为通过返回的index位置去检索具体的文档，并且在内存中进行了排序（stage有多个模式，将在后文中进行解释）。
     *  queryPlanner.winningPlan.inputStage:用来描述子stage，并且为其父stage提供文档和索引关键字。
     *  queryPlanner.winningPlan.inputStage.stage: 此处是SORT\_KEY\_GENERATOR，[这个在官网上找了好久只找到一个issues][issues]，[最后在stackoverflow上找到了我需要的答案][stackoverflow]，最关键的一句话： means in-memory sorting，在内存中排序的
     *  queryPlanner.winningPlan.keyPattern:所扫描的index内容，此处是"subscriberID" : 1,“momentID” : 1
     *  queryPlanner.winningPlan.indexName：winning plan所选用的index。这里选用的idx\_objectid\_momentid 索引
     *  queryPlanner.winningPlan.isMultiKey是否是Multikey，此处返回是false，如果索引建立在array上，此处将是true。
     *  queryPlanner.winningPlan.direction：此query的查询顺序，此处是forward，如果查询顺序与设置顺序相反则必然是backward，此部分以下会用案例解释。
     *  queryPlanner.winningPlan.indexBounds:winningplan所扫描的索引范围,如果没有制定范围就是\[MaxKey, MinKey\]，这主要是直接定位到mongodb的chunck中去查找数据，加快数据读取。（chunk是指一个集合数据中的子集，在分片的时候，每个分区就是一个chunck）
     *  queryPlanner.rejectedPlans：其他执行计划（非最优而被查询优化器reject的）的详细返回，其中具体信息与winningPlan的返回中意义相同
 *  对executionStats中时间分析
    
     *  executionTimeMillis : 指我们预计的执行时间，越小越好，其中有三个这个时间数据，分别如下：
        
         *  第一层， executionStats.executionTimeMillis： 该query的整体查询时间。
         *  第二层，executionStats.executionStages.executionTimeMillisEstimate： 该查询根据index去检索document获得71条数据的时间。
         *  第三层，executionStats.executionStages.inputStage.executionTimeMillisEstimate：此处没有，该查询扫描71行index所用时间。
 *  对executionStats中扫描数量的分析
    
     *  nReturned、totalKeysExamined、totalDocsExamined，分别代表查询返回条数，索引扫描条数，文档扫描条数，这些都是直观地影响到executionTimeMillis，我们需要扫描的越少速度越快。
     *  对应一个查询，理想状态下是 nReturned=totalKeysExamined=totalDocsExamined，也就是我们需要的数据都在索引数据文档中找到了
 *  对stage状态分析
    
     *  stage的类型会影响totalKeysExamined和totalDocsExamined的最终扫描值
     *  COLLSCAN：全表扫描
     *  IXSCAN：索引扫描
     *  FETCH：根据索引去检索指定document
     *  SHARD\_MERGE：将各个分片返回数据进行merge
     *  SORT：表明在内存中进行了排序
     *  SORT\_KEY\_GENERATOR ： 表名在内存中排序
     *  LIMIT：使用limit限制返回数
     *  SKIP：使用skip进行跳过
     *  IDHACK：针对\_id进行查询
     *  SHARDING\_FILTER：通过mongos对分片数据进行查询
     *  COUNT：利用db.coll.explain().count()之类进行count运算
     *  COUNTSCAN：count不使用Index进行count时的stage返回
     *  COUNT\_SCAN：count使用了Index进行count时的stage返回
     *  SUBPLA：未使用到索引的$or查询的stage返回
     *  TEXT：使用全文索引进行查询时候的stage返回
     *  PROJECTION：限定返回字段时候stage的返回
     *  SINGLE\_SHARD： 查询落到单个分片上查询
 *  我们生产中期望得到的是：
    
     *  Fetch+IDHACK
     *  Fetch+IXSCAN
     *  Limit+（Fetch+IXSCAN）
     *  PROJECTION+IXSCAN
     *  SINGLE\_SHARD+IXSCAN
     *  COUNT\_SCAN
 *  不希望看到的：
    
     *  COLLSCAN，全表
     *  SORT，使用排序但是没有命中索引，内存排序
     *  不要用SKIP去做跳过操作
     *  SUBPLA，没有用到索引的or命令
     *  COUNTSCAN，没有用到索引的count
 *  关于上面direction是backward的原因，有必要说明：
 *  我们在没有设置排序字段时候，因为走的索引是idx\_objectid\_momentid ：\{“subscriberID” : 1,“momentID” : 1\}，两个字段索引都是顺序创建的。如下在默认无指定排序情况下，momentID是顺序排列的。  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 2]
 *  如果我们强制这条语句走 subscriberID\_1\_type\_1\_auditTime\_1 索引，那么他肯定是backround，因为你查询的顺序和你索引创建的顺序正好是相反的：

//db.moment.explain("executionStats").find({"subscriberID" : NumberLong(1884831336), "type" : { "$in" : [1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] } }).sort({"auditTime": -1}).hint({"subscriberID" : 1,"type" : 1,"auditTime" : 1});
    { 
    	"queryPlanner" : { 
    		"mongosPlannerVersion" : 1,
    		"winningPlan" : { 
    			"stage" : "SINGLE_SHARD",
    			"shards" : [
    				{ 
    					"shardName" : "cmgo-hgz5ixhj_0",
    					"connectionString" : "cmgo-hgz5ixhj_0/100.120.151.75:7004,100.120.152.197:7000,100.93.47.81:7016",
    					"serverInfo" : { 
    						"host" : "TENCENT64.site",
    						"port" : 7016,
    						"version" : "3.2.10-cmongo-",
    						"gitVersion" : "nogitversion"
    					},
    					"plannerVersion" : 1,
    					"namespace" : "moment.moment",
    					"indexFilterSet" : false,
    					"parsedQuery" : { 
    						"$and" : [
    							{ 
    								"subscriberID" : { 
    									"$eq" : NumberLong(1884831336)
    								}
    							},
    							{ 
    								"type" : { 
    									"$in" : [
    										1,
    										2,
    										3,
    										4,
    										5,
    										6,
    										7,
    										8,
    										10,
    										11,
    										12,
    										13,
    										14,
    										15,
    										16,
    										17,
    										18,
    										19
    									]
    								}
    							}
    						]
    					},
    					"winningPlan" : { 
    						"stage" : "FETCH",
    						"inputStage" : { 
    							"stage" : "SORT_MERGE",
    							"sortPattern" : { 
    								"auditTime" : -1
    							},
    							"inputStages" : [
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[19.0, 19.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[18.0, 18.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[17.0, 17.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[16.0, 16.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[15.0, 15.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[14.0, 14.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[13.0, 13.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[12.0, 12.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[11.0, 11.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[10.0, 10.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[8.0, 8.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[7.0, 7.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[6.0, 6.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[5.0, 5.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[4.0, 4.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[3.0, 3.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[2.0, 2.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									}
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[1.0, 1.0]"
    										],
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    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[3.0, 3.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									},
    									"keysExamined" : 25,
    									"dupsTested" : 0,
    									"dupsDropped" : 0,
    									"seenInvalidated" : 0
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"nReturned" : 28,
    									"executionTimeMillisEstimate" : 0,
    									"works" : 29,
    									"advanced" : 28,
    									"needTime" : 0,
    									"needYield" : 0,
    									"saveState" : 1,
    									"restoreState" : 1,
    									"isEOF" : 1,
    									"invalidates" : 0,
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[2.0, 2.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									},
    									"keysExamined" : 28,
    									"dupsTested" : 0,
    									"dupsDropped" : 0,
    									"seenInvalidated" : 0
    								},
    								{ 
    									"stage" : "IXSCAN",
    									"nReturned" : 1,
    									"executionTimeMillisEstimate" : 0,
    									"works" : 2,
    									"advanced" : 1,
    									"needTime" : 0,
    									"needYield" : 0,
    									"saveState" : 1,
    									"restoreState" : 1,
    									"isEOF" : 1,
    									"invalidates" : 0,
    									"keyPattern" : { 
    										"subscriberID" : 1,
    										"type" : 1,
    										"auditTime" : 1
    									},
    									"indexName" : "subscriberID_1_type_1_auditTime_1",
    									"isMultiKey" : false,
    									"isUnique" : false,
    									"isSparse" : false,
    									"isPartial" : false,
    									"indexVersion" : 1,
    									"direction" : "backward",
    									"indexBounds" : { 
    										"subscriberID" : [
    											"[1884831336, 1884831336]"
    										],
    										"type" : [
    											"[1.0, 1.0]"
    										],
    										"auditTime" : [
    											"[MaxKey, MinKey]"
    										]
    									},
    									"keysExamined" : 1,
    									"dupsTested" : 0,
    									"dupsDropped" : 0,
    									"seenInvalidated" : 0
    								}
    							]
    						}
    					}
    				}
    			]
    		}
    	},
    	"ok" : 1
    }

*  以上语句也就是我们动态中用的第二多的一条语句，当关注页面查看第二页动态时候，会带上\_id，这个时候我们用hint 命令强制他走了 subscriberID\_1\_type\_1\_auditTime\_1 索引，此时他的查询是FETCH + SORT\_MERGE，其中SORT\_MERGE 也是在内存进行排序并且合并得到最好结果。
 *  以上分析起来索引应该有优化的空间，例如我建立以下索引：

db.moment.createIndex({ "subscriberID" : 1,"type" : 1,"auditTime" : -1 })

*  看起来完美契合我们的查询：

db.moment.explain("executionStats").find({  "subscriberID" : NumberLong(2000103230), "type" : {  "$in" : [1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] } }).sort({ "auditTime": -1});

//得到的执行结果：
    { 
        "queryPlanner" : { 
            "plannerVersion" : 1,
            "namespace" : "moment.moment",
            "indexFilterSet" : false,
            "parsedQuery" : { 
                "$and" : [
                    { 
                        "subscriberID" : { 
                            "$eq" : NumberLong(2000103230)
                        }
                    },
                    { 
                        "type" : { 
                            "$in" : [
                                1,
                                2,
                                3,
                                4,
                                5,
                                6,
                                7,
                                8,
                                10,
                                11,
                                12,
                                13,
                                14,
                                15,
                                16,
                                17,
                                18,
                                19
                            ]
                        }
                    }
                ]
            },
            "winningPlan" : { 
                "stage" : "SORT",
                "sortPattern" : { 
                    "auditTime" : -1
                },
                "inputStage" : { 
                    "stage" : "SORT_KEY_GENERATOR",
                    "inputStage" : { 
                        "stage" : "FETCH",
                        "filter" : { 
                            "type" : { 
                                "$in" : [
                                    1,
                                    2,
                                    3,
                                    4,
                                    5,
                                    6,
                                    7,
                                    8,
                                    10,
                                    11,
                                    12,
                                    13,
                                    14,
                                    15,
                                    16,
                                    17,
                                    18,
                                    19
                                ]
                            }
                        },
                        "inputStage" : { 
                            "stage" : "IXSCAN",
                            "keyPattern" : { 
                                "subscriberID" : 1,
                                "unread" : 1
                            },
                            "indexName" : "subscriberID_1_unread_1",
                            "isMultiKey" : false,
                            "multiKeyPaths" : { 
                                "subscriberID" : [ ],
                                "unread" : [ ]
                            },
                            "isUnique" : false,
                            "isSparse" : false,
                            "isPartial" : false,
                            "indexVersion" : 2,
                            "direction" : "forward",
                            "indexBounds" : { 
                                "subscriberID" : [
                                    "[2000103230, 2000103230]"
                                ],
                                "unread" : [
                                    "[MinKey, MaxKey]"
                                ]
                            }
                        }
                    }
                }
            },
            "rejectedPlans" : [
                { 
                    "stage" : "FETCH",
                    "inputStage" : { 
                        "stage" : "SORT_MERGE",
                        "sortPattern" : { 
                            "auditTime" : -1
                        },
                        "inputStages" : [
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[1.0, 1.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[2.0, 2.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[3.0, 3.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[4.0, 4.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[5.0, 5.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[6.0, 6.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[7.0, 7.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[8.0, 8.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[10.0, 10.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[11.0, 11.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[12.0, 12.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[13.0, 13.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[14.0, 14.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[15.0, 15.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[16.0, 16.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[17.0, 17.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
                                        "[18.0, 18.0]"
                                    ],
                                    "auditTime" : [
                                        "[MaxKey, MinKey]"
                                    ]
                                }
                            },
                            { 
                                "stage" : "IXSCAN",
                                "keyPattern" : { 
                                    "subscriberID" : 1,
                                    "type" : 1,
                                    "auditTime" : -1
                                },
                                "indexName" : "subscriberID_1_type_1_auditTime_-1",
                                "isMultiKey" : false,
                                "multiKeyPaths" : { 
                                    "subscriberID" : [ ],
                                    "type" : [ ],
                                    "auditTime" : [ ]
                                },
                                "isUnique" : false,
                                "isSparse" : false,
                                "isPartial" : false,
                                "indexVersion" : 2,
                                "direction" : "forward",
                                "indexBounds" : { 
                                    "subscriberID" : [
                                        "[2000103230, 2000103230]"
                                    ],
                                    "type" : [
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            "version" : "3.4.21",
            "gitVersion" : "ba2e818de44c9a331ce09a624780a2bd841cf6e4"
        },
        "ok" : 1
    }

*  我们看到结果依然是 SORT + SORT\_KEY\_GENERATOR，意思是我们依然需要在内存中排序，原因在于当我们建立：\{“subscriberID” : 1,“type” : 1,“auditTime” : -1 \} 索引的时候，MongoDB会按照如下方式给我吗组织索引数据：
    
     *  优先按subscriberID 的顺序，
     *  然后 type顺序
     *  最后才是 auditTime 的逆序，如下案例

mongos> db.moment.find({  "subscriberID" : NumberLong(1896740278), "type" : {  "$in" : [1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] } });
    {  "_id" : ObjectId("5fd09738fb1a7b001444e64b"), "type" : 1, "momentID" : NumberLong(257617647), "memberID" : NumberLong(1519251653), "subscriberID" : NumberLong(1896740278), "auditStatus" : 0, "unread" : true, "createTime" : ISODate("2020-12-09T09:21:59Z"),  }
    {  "_id" : ObjectId("60a3a82062c4680014c7dd21"), "type" : 1, "momentID" : NumberLong(405945302), "subscriberID" : NumberLong(1896740278), "unread" : true, "auditTime" : ISODate("2021-05-18T11:42:25Z"), "createTime" : ISODate("2021-05-18T11:28:11Z"),  }
    {  "_id" : ObjectId("60a65571d359c60014c6149c"), "type" : 1, "momentID" : NumberLong(406114684), "subscriberID" : NumberLong(1896740278), "unread" : true, "auditTime" : ISODate("2021-05-20T12:26:25Z"), "createTime" : ISODate("2021-05-20T12:10:39Z"),  }
    {  "_id" : ObjectId("60d3000178512e001443830f"), "type" : 1, "momentID" : NumberLong(408785569), "subscriberID" : NumberLong(1896740278), "unread" : true, "auditTime" : ISODate("2021-06-23T09:33:53Z"), "createTime" : ISODate("2021-06-23T09:26:52Z"),  }
    {  "_id" : ObjectId("60e6d944ccc7d000148edac2"), "type" : 1, "momentID" : NumberLong(409971805), "subscriberID" : NumberLong(1896740278), "unread" : true, "auditTime" : ISODate("2021-07-08T10:53:56Z"), "createTime" : ISODate("2021-07-08T10:45:38Z"),  }
    {  "_id" : ObjectId("60eb97d3f12ad4001491f83f"), "type" : 1, "momentID" : NumberLong(410215242), "subscriberID" : NumberLong(1896740278), "unread" : true, "auditTime" : ISODate("2021-07-12T01:16:03Z"), "createTime" : ISODate("2021-07-12T01:07:27Z"),  }
    {  "_id" : ObjectId("61016cc1b572ea001497550f"), "type" : 1, "momentID" : NumberLong(411549007), "subscriberID" : NumberLong(1896740278), "unread" : true, "auditTime" : ISODate("2021-07-28T14:42:09Z"), "createTime" : ISODate("2021-07-28T14:42:09Z"),  }

*  可以看到在objectId相同的时候，他是按照momentID的顺序排列，但是auditTime的顺序只有在momentID相同的时候才是逆序，实际按索引来看他还是乱序的。因此我们在对时间排序的时候，他依然需要在内存排序。
 *  那么我们应该创建如下的索引：

db.moment.createIndex({ "subscriberID" : 1,"auditTime" : -1 ,"type" : 1})         
    db.moment.createIndex({ "type" : 1,"auditTime" : -1,"subscriberID" : 1})

*  因为我们是分片的数据库，按照subscriberID分片，因此，这两个索引效果是一样的。但是两个都创建的话，MongoDB会给你选择第一个，排序优先
 *  在我们项目中，查询关注动态部分有两个查询语句，一条查询第一页，一条查询之后的页面数据，带有\_id，占比如下：
 *  总的查询数：2,007,778

![请添加图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 3]

*  第二页以及以后的查询数：584258

![请添加图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 4]

*  第二页查询占比28% 左右，也就是大多其实都是在第一页查看，由此看来索引还是有优化空间的。

[9ca839a5dc794b7b9dae5abf49c8ae1b.png]: /images/20220829/b3f5bcb9cd4845f79a8219b24d4d826c.png
[e5a62ee3b1a743da841fbf8148a9fda2.png]: /images/20220829/023633f8a17e4c6d9ef240c17b285837.png
[b1d24971ce284f249be8716b80af3811.png]: /images/20220829/8244def9be5c43aeaf826c01526ef24a.png
[Mongodb_explain]: https://docs.mongodb.com/manual/reference/method/db.collection.explain/
[Mongodb_explain_]: https://docs.mongoing.com/indexes/measure-index-use#id2
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70]: /images/20220829/9f336d029a4d47d39cc6cef0b470f1c1.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 1]: /images/20220829/996a4b0577ef4ff8b387cb7e74e45fdb.png
[issues]: https://jira.mongodb.org/browse/SERVER-33895
[stackoverflow]: https://stackoverflow.com/questions/66438683/mongo-sort-key-generator-stage-though-there-is-a-proper-index
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 2]: /images/20220829/a2968037fc1c460ba1f420af2e07e4a8.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 3]: /images/20220829/5c313d8caea2429d99451c3f08238f0b.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xpYW9qaWFtaW4wMTAy_size_16_color_FFFFFF_t_70 4]: /images/20220829/a085004f4a16462b9d88b91138ac0c40.png