Mongo低版本 count操作进行$in时走了覆盖索引却仍然回表

文章目录

- 概要
- 一、原因
- 二、解决方法

概要

由于历史原因，一些老项目还在用MongoDB V3.2版本，集群为分片模式，一个客户表数量有15亿左右，昨天监控突然报很多慢查询，如下：

db.info.count({domain_id:888,status:{$in:[2,3]}})

表结构关键字段如下：

字段	类型	注释
domain_id	int64	公司id
customer_id	int64	公司下的客户id
status	int32	客户状态

索引如下：

db.info.createIndex({domain_id:1,status:1,modify_time:1})

一、原因

遇到慢查询，当然要explain下看下查询语句执行状况了，如下：

db.info.explain('executionStats').count({domain_id:4730422,status:{$in:[2,3]}})
{"queryPlanner" : {"plannerVersion" : 1,"namespace" : "customer.info","indexFilterSet" : false,"parsedQuery" : {"$and" : [{"domain_id" : {"$eq" : 4730422}},{"status" : {"$in" : [2,3]}}]},"winningPlan" : {"stage" : "COUNT","inputStage" : {"stage" : "FETCH","inputStage" : {"stage" : "IXSCAN","keyPattern" : {"domain_id" : 1,"status" : 1,"modify_time" : -1},"indexName" : "domain_id_1_status_1_modify_time_-1","isMultiKey" : false,"direction" : "forward","indexBounds" : {"domain_id" : ["[4730422.0, 4730422.0]"],"status" : ["[2.0, 2.0]","[3.0, 3.0]"],"modify_time" : ["[MaxKey, MinKey]"]}}}},"rejectedPlans" : []},"executionStats" : {"executionSuccess" : true,"nReturned" : 0,"executionTimeMillis" : 0,"totalKeysExamined" : 9,   #索引扫描个数"totalDocsExamined" : 9,   #文档扫描个数，不等于0表示有回表"executionStages" : {"stage" : "COUNT","nReturned" : 0,"executionTimeMillisEstimate" : 0,"works" : 10,"advanced" : 0,"needTime" : 9,"needFetch" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"nCounted" : 9,"nSkipped" : 0,"inputStage" : {"stage" : "FETCH","nReturned" : 9,"executionTimeMillisEstimate" : 0,"works" : 10,"advanced" : 9,"needTime" : 0,"needFetch" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"docsExamined" : 9,"alreadyHasObj" : 0,"inputStage" : {"stage" : "IXSCAN","nReturned" : 9,"executionTimeMillisEstimate" : 0,"works" : 10,"advanced" : 9,"needTime" : 0,"needFetch" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"keyPattern" : {"domain_id" : 1,"status" : 1,"modify_time" : -1},"indexName" : "domain_id_1_status_1_modify_time_-1","isMultiKey" : false,"direction" : "forward","indexBounds" : {"domain_id" : ["[4730422.0, 4730422.0]"],"status" : ["[2.0, 2.0]","[3.0, 3.0]"],"modify_time" : ["[MaxKey, MinKey]"]},"keysExamined" : 9,"dupsTested" : 0,"dupsDropped" : 0,"seenInvalidated" : 0,"matchTested" : 0}}} },"ok" : 1
}

通过上面索引执行分析可以看到 totalDocsExamined不为0，所以进行了回表，理论上走了覆盖索引是不用回表的。
我们下面再试试没有$in的情况下，查询分析：

db.info.explain('executionStats').count({domain_id:4730422,status:3})
{"queryPlanner" : {"plannerVersion" : 1,"namespace" : "customer.info","indexFilterSet" : false,"parsedQuery" : {"$and" : [{"domain_id" : {"$eq" : 4730422}},{"status" : {"$eq" : 3}}]},"winningPlan" : {"stage" : "COUNT","inputStage" : {"stage" : "COUNT_SCAN","keyPattern" : {"domain_id" : 1,"status" : 1,"modify_time" : -1},"indexName" : "domain_id_1_status_1_modify_time_-1","isMultiKey" : false}},"rejectedPlans" : [ ]},"executionStats" : {"executionSuccess" : true,"nReturned" : 0,"executionTimeMillis" : 0,"totalKeysExamined" : 8,"totalDocsExamined" : 0,"executionStages" : {"stage" : "COUNT","nReturned" : 0,"executionTimeMillisEstimate" : 0,"works" : 8,"advanced" : 0,"needTime" : 7,"needFetch" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"nCounted" : 6,"nSkipped" : 0,"inputStage" : {"stage" : "COUNT_SCAN","nReturned" : 6,"executionTimeMillisEstimate" : 0,"works" : 7,"advanced" : 6,"needTime" : 1,"needFetch" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"invalidates" : 0,"keysExamined" : 8,"keyPattern" : {"domain_id" : 1,"status" : 1,"modify_time" : -1},"indexName" : "domain_id_1_status_1_modify_time_-1","isMultiKey" : false}},"allPlansExecution" : [ ]},"ok" : 1
}

可以看到此时totalDocsExamined的值为0，说明不使用$in操作就不会回表了。这显然是count操作在$in时的bug。
我们用docker快速搭建一个MongoDB V4.4的副本集试试，如下：

rsm:PRIMARY> db.info.explain('executionStats').count({domain_id:15600,status:{$in:[2,3]}})
{"queryPlanner" : {"plannerVersion" : 1,"namespace" : "customer.info","indexFilterSet" : false,"parsedQuery" : {"$and" : [{"domain_id" : {"$eq" : 15600}},{"status" : {"$in" : [2,3]}}]},"winningPlan" : {"stage" : "COUNT","inputStage" : {"stage" : "IXSCAN","keyPattern" : {"domain_id" : 1,"status" : 1},"indexName" : "domain_id_1_status_1","isMultiKey" : false,"multiKeyPaths" : {"domain_id" : [ ],"status" : [ ]},"isUnique" : false,"isSparse" : false,"isPartial" : false,"indexVersion" : 2,"direction" : "forward","indexBounds" : {"domain_id" : ["[15600.0, 15600.0]"],"status" : ["[2.0, 2.0]","[3.0, 3.0]"]}}},"rejectedPlans" : [ ]},"executionStats" : {"executionSuccess" : true,"nReturned" : 0,"executionTimeMillis" : 0,"totalKeysExamined" : 5,"totalDocsExamined" : 0, "executionStages" : {"stage" : "COUNT","nReturned" : 0,"executionTimeMillisEstimate" : 0,"works" : 6,"advanced" : 0,"needTime" : 5,"needYield" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"nCounted" : 5,"nSkipped" : 0,"inputStage" : {"stage" : "IXSCAN","nReturned" : 5,"executionTimeMillisEstimate" : 0,"works" : 6,"advanced" : 5,"needTime" : 0,"needYield" : 0,"saveState" : 0,"restoreState" : 0,"isEOF" : 1,"keyPattern" : {"domain_id" : 1,"status" : 1},"indexName" : "domain_id_1_status_1","isMultiKey" : false,"multiKeyPaths" : {"domain_id" : [ ],"status" : [ ]},"isUnique" : false,"isSparse" : false,"isPartial" : false,"indexVersion" : 2,"direction" : "forward","indexBounds" : {"domain_id" : ["[15600.0, 15600.0]"],"status" : ["[2.0, 2.0]","[3.0, 3.0]"]},"keysExamined" : 5,"seeks" : 1,"dupsTested" : 0,"dupsDropped" : 0}}},"serverInfo" : {"host" : "b8b178041a4e","port" : 27021,"version" : "4.4.28","gitVersion" : "61c2baf63a060f7c12bd76e779044800ae18710b"},"ok" : 1,"$clusterTime" : {"clusterTime" : Timestamp(1711857956, 1),"signature" : {"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),"keyId" : NumberLong(0)}},"operationTime" : Timestamp(1711857956, 1)
}