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。
  • 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，中文的Mongodb中explain() 网址

  • 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：

  • 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，最后在stackoverflow上找到了我需要的答案，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。

  • 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" : 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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：
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false,"isPartial" : false,"indexVersion" : 2,"direction" : "backward","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" : "backward","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" : "backward","indexBounds" : {"subscriberID" : ["[2000103230, 2000103230]"],"type" : ["[1.0, 1.0]"],"auditTime" : ["[MaxKey, MinKey]"]}}]}}]},"executionStats" : {"executionSuccess" : true,"nReturned" : 4,"executionTimeMillis" : 1,"totalKeysExamined" : 4,"totalDocsExamined" : 4,"executionStages" : {"stage" : "SORT","nReturned" : 4,"executionTimeMillisEstimate" : 0,"works" : 12,"advanced" : 4,"needTime" : 6,"needYield" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"sortPattern" : {"auditTime" : -1},"memUsage" : 764,"memLimit" : 33554432,"inputStage" : {"stage" : "SORT_KEY_GENERATOR","nReturned" : 4,"executionTimeMillisEstimate" : 0,"works" : 6,"advanced" : 4,"needTime" : 1,"needYield" : 0,"saveState" : 0,"restoreState" : 0,"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" : 4,"executionTimeMillisEstimate" : 0,"works" : 5,"advanced" : 4,"needTime" : 0,"needYield" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"docsExamined" : 4,"alreadyHasObj" : 0,"inputStage" : {"stage" : "IXSCAN","nReturned" : 4,"executionTimeMillisEstimate" : 0,"works" : 5,"advanced" : 4,"needTime" : 0,"needYield" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"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]"]},"keysExamined" : 4,"seeks" : 1,"dupsTested" : 0,"dupsDropped" : 0,"seenInvalidated" : 0}}}}},"serverInfo" : {"host" : "mongodb-0","port" : 27017,"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

  • 第二页以及以后的查询数：584258

  • 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。

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