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MongoDB 索引的最佳实践

步履不停
发布: 2019-06-24 17:36:15
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MongoDB 索引的最佳实践

前言

大部分开发者都知道加索引会快。但实际过程中,我们常碰到一些疑问&困难:

  • 我们查询的字段会各种case都有,是不是各个涉及查询的字段都要加索引?
  • 复合索引和单字段怎么选择,都加还是每一个的单个字段就好了?
  • 加索引有没有副作用?
  • 索引都加了,但还是不够快?怎么办?

本文尝试去解释索引的基本知识&解答上述的疑问。

一、索引究竟是什么东西?

大部分开发者接触索引,大概知道索引类似书的目录,你要找到想要的内容,通过目录找到限定的关键字,进而找到对应的章节的pageno,再找到具体的内容。
在数据结构里面,最简单的索引实现类似hashmap,通过关键字key,映射到具体的位置,找到具体的内容。但除了hash的方式,还有多种的方式实现索引。

(一)索引的多种实现方式以及特色

hash / b-tree / b+-tree redis HSET / MongoDB&PostgreSQL / MySQL

hashmap

bca2bb3a-4c85-4b43-aba5-6c12f9381a6b.jpg

一图见b-tree & b+-tree 差别:

69ff4243-f3fd-4b9e-93ce-35cf9dd7d775.jpg

  • b+-tree 叶子存数据,非叶子存索引,不存数据,叶子间有link
  • b-tree 非叶子可存数据

算法查找复杂度上来说:

  • hash 接近O(1)
  • b-tree O(1)~ O(Log(n))更快的平均查找时间,不稳定的查询时间
  • b+ tree O(Log(n)) 连续数据, 查询的稳定性

至于为何MongoDB 的实现选择b-tree 而非 b+-tree ?
网上多篇文章有阐述,非本文重点。

(二)数据&索引的存储

74b31b12-fe8a-46e5-b30f-a375d8ba29a9.jpgindex尽量存储在内存,data 其次。
注意只保留必要的index,内存尽量用在刀刃上。
如果index memory 都接近占满memory,那么就很容易读到disk,速度就下来了。

(三)知道索引的实现&存储原理后的思考

insert/update/delete 会触发rebalance tree,所以,增删改数据,索引会触发修改,性能会有损耗,索引不是越多越好。既然如此,选哪些字段作为索引呢?当查询用到这些条件,怎么办?
拿一个最简单的hashmap来讲,为什么复杂度不是O(1),而是所谓接近 O(1)。因为有key 冲突/重复,DB 去找的时候,key 冲突的数据一大堆的话,还是得轮着继续找。b-tree 看键(key)的选择也是如此。
因此一个大部分开发经常犯的错就是对没有区分度的key建索引。例如:很多就只有集中类别的 type/status 的 documents count 达几十万以上的collection,通常这种索引没什么帮助。

二、复合索引

(一)复合索引不是越多越好

如果不想多建多余的索引,开发的同事在复合 & 单个字段选择上有时候挺纠结的。 根据典型碰到的场景,来做几个实验:
这里创建了个loans collection。简化只有100条数据。这个是借贷的表有 _id, userId, status(借贷状态), amount(金额).

db.loans.count()100

db.loans.find({ "userId" : "59e022d33f239800129c61c7", "status" : "repayed", }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ { "status" : { "$eq" : "repayed" } }, { "userId" : { "$eq" : "59e022d33f239800129c61c7" } } ] }, "queryHash" : "15D5A9A1", "planCacheKey" : "15D5A9A1", "winningPlan" : { "stage" : "COLLSCAN", "filter" : { "$and" : [ { "status" : { "$eq" : "repayed" } }, { "userId" : { "$eq" : "59e022d33f239800129c61c7" } } ] }, "direction" : "forward" }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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注意上面 COLLSCAN 全表扫描了,因为没有索引。接下来我们分别建立几个索引。
step 1 先建立 {userId:1, status:1}

db.loans.createIndex({userId:1, status:1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 }
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db.loans.find({ "userId" : "59e022d33f239800129c61c7", "status" : "repayed", }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ { "status" : { "$eq" : "repayed" } }, { "userId" : { "$eq" : "59e022d33f239800129c61c7" } } ] }, "queryHash" : "15D5A9A1", "planCacheKey" : "BB87F2BA", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ], "status" : [ "["repayed", "repayed"]" ] } } }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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结果:如愿命中 {userId:1, status:1} 作为 winning plan。

step2:再建立个典型的索引 userId

db.loans.createIndex({userId:1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 2, "numIndexesAfter" : 3, "ok" : 1 }
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db.loans.find({ "userId" : "59e022d33f239800129c61c7", "status" : "repayed", }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ { "status" : { "$eq" : "repayed" } }, { "userId" : { "$eq" : "59e022d33f239800129c61c7" } } ] }, "queryHash" : "15D5A9A1", "planCacheKey" : "1B1A4861", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "[\"59e022d33f239800129c61c7\", \"59e022d33f239800129c61c7\"]" ], "status" : [ "[\"repayed\", \"repayed\"]" ] } } }, "rejectedPlans" : [ { "stage" : "FETCH", "filter" : { "status" : { "$eq" : "repayed" } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1 }, "indexName" : "userId_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ] } } } ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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留意到 DB 检测到 {userId:1, status:1} 为更优执行的方案.

db.loans.find({ "userId" : "59e022d33f239800129c61c7" }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "userId" : { "$eq" : "59e022d33f239800129c61c7" } }, "queryHash" : "B1777DBA", "planCacheKey" : "1F09D68E", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1 }, "indexName" : "userId_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ] } } }, "rejectedPlans" : [ { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ], "status" : [ "[MinKey, MaxKey]" ] } } } ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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留意到 DB 检测到 {userId:1} 为更优执行的方案,嗯~,如我们所料.

db.loans.find({ "status" : "repayed" }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "status" : { "$eq" : "repayed" } }, "queryHash" : "E6304EB6", "planCacheKey" : "7A94191B", "winningPlan" : { "stage" : "COLLSCAN", "filter" : { "status" : { "$eq" : "repayed" } }, "direction" : "forward" }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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有趣的部分:status不命中索引,全表扫描
接下来的步骤,加个sort :

db.loans.find({ "userId" : "59e022d33f239800129c61c7" }).sort({status:1}).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "userId" : { "$eq" : "59e022d33f239800129c61c7" } }, "queryHash" : "F5ABB1AA", "planCacheKey" : "764CBAA8", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ], "status" : [ "[MinKey, MaxKey]" ] } } }, "rejectedPlans" : [ { "stage" : "SORT", "sortPattern" : { "status" : 1 }, "inputStage" : { "stage" : "SORT_KEY_GENERATOR", "inputStage" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1 }, "indexName" : "userId_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ] } } } } } ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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(二)其他尝试

有趣的部分:status 不命中索引

db.loans.find({ "status" : "repayed","userId" : "59e022d33f239800129c61c7", }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ { "status" : { "$eq" : "repayed" } }, { "userId" : { "$eq" : "59e022d33f239800129c61c7" } } ] }, "queryHash" : "15D5A9A1", "planCacheKey" : "1B1A4861", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "[\"59e022d33f239800129c61c7\", \"59e022d33f239800129c61c7\"]" ], "status" : [ "[\"repayed\", \"repayed\"]" ] } } }, "rejectedPlans" : [ { "stage" : "FETCH", "filter" : { "status" : { "$eq" : "repayed" } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1 }, "indexName" : "userId_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ] } } } ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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命中索引,跟query的各个字段顺序不相关,如我们猜测。
有趣部分再来, 我们删掉索引{userId:1}

db.loans.dropIndex({"userId":1}) { "nIndexesWas" : 3, "ok" : 1 } db.loans.find({"userId" : "59e022d33f239800129c61c7", }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "userId" : { "$eq" : "59e022d33f239800129c61c7" } }, "queryHash" : "B1777DBA", "planCacheKey" : "5776AB9C", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]" ], "status" : [ "[MinKey, MaxKey]" ] } } }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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DB 执行分析器觉得索引{userId:1, status:1} 能更优,没有命中复合索引,这个是因为status不是leading field。

db.loans.find({ "status" : "repayed" }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "status" : { "$eq" : "repayed" } }, "queryHash" : "E6304EB6", "planCacheKey" : "7A94191B", "winningPlan" : { "stage" : "COLLSCAN", "filter" : { "status" : { "$eq" : "repayed" } }, "direction" : "forward" }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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再换个角度sort 一遍, 与前面query & sort互换,之前是:

db.loans.find({userId:1}).sort({ "status" : "repayed" })
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看看有啥不一样?

db.loans.find({ "status" : "repayed" }).sort({userId:1}).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "status" : { "$eq" : "repayed" } }, "queryHash" : "56EA6313", "planCacheKey" : "2CFCDA7F", "winningPlan" : { "stage" : "FETCH", "filter" : { "status" : { "$eq" : "repayed" } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "userId" : 1, "status" : 1 }, "indexName" : "userId_1_status_1", "isMultiKey" : false, "multiKeyPaths" : { "userId" : [ ], "status" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "userId" : [ "[MinKey, MaxKey]" ], "status" : [ "[MinKey, MaxKey]" ] } } }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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如猜测,命中索引。
再来玩玩,确认下leading filed试验:

db.loans.dropIndex("userId_1_status_1") { "nIndexesWas" : 2, "ok" : 1 }
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db.loans.getIndexes() [ { "v" : 2, "key" : { "id" : 1 }, "name" : "id_", "ns" : "cashLoan.loans" } ]
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db.loans.createIndex({status:1, userId:1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 }
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db.loans.getIndexes() [ { "v" : 2, "key" : { "id" : 1 }, "name" : "id_", "ns" : "cashLoan.loans" }, { "v" : 2, "key" : { "status" : 1, "userId" : 1 }, "name" : "status_1_userId_1", "ns" : "cashLoan.loans" } ]
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db.loans.find({ "status" : "repayed" }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "status" : { "$eq" : "repayed" } }, "queryHash" : "E6304EB6", "planCacheKey" : "7A94191B", "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "status" : 1, "userId" : 1 }, "indexName" : "status_1_userId_1", "isMultiKey" : false, "multiKeyPaths" : { "status" : [ ], "userId" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "status" : [ "["repayed", "repayed"]" ], "userId" : [ "[MinKey, MaxKey]" ] } } }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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db.loans.getIndexes() [ { "v" : 2, "key" : { "id" : 1 }, "name" : "id_", "ns" : "cashLoan.loans" }, { "v" : 2, "key" : { "status" : 1, "userId" : 1 }, "name" : "status_1_userId_1", "ns" : "cashLoan.loans" } ]
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db.loans.find({"userId" : "59e022d33f239800129c61c7", }).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "cashLoan.loans", "indexFilterSet" : false, "parsedQuery" : { "userId" : { "$eq" : "59e022d33f239800129c61c7" } }, "queryHash" : "B1777DBA", "planCacheKey" : "5776AB9C", "winningPlan" : { "stage" : "COLLSCAN", "filter" : { "userId" : { "$eq" : "59e022d33f239800129c61c7" } }, "direction" : "forward" }, "rejectedPlans" : [ ] }, "serverInfo" : { "host" : "RMBAP", "port" : 27017, "version" : "4.1.11", "gitVersion" : "1b8a9f5dc5c3314042b55e7415a2a25045b32a94" }, "ok" : 1 }
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看完这个试验,明白了 {userId:1, status:1} vs {status:1,userId:1} 的差别了吗?

PS:这个case 里面其实status 区分度不高,这里只是作为实例展示。

三、总结:

  • 注意使用上、使用频率上、区分高的/常用的在前面;
  • 如果需要减少索引以节省memory/提高修改数据的性能的话,可以保留区分度高,常用的,去除区分度不高,不常用的索引。
  • 学会用explain()验证分析性能:

DB 一般都有执行器优化的分析,MySQL & MongoDB 都是 用explain 来做分析。
语法上MySQL :

explain your_sql

MongoDB:

yoursql.explain()

总结典型:理想的查询是结合explain 的指标,他们通常是多个的混合:

  • IXSCAN : 索引命中;
  • Limit : 带limit;
  • Projection : 相当于非 select * ;
  • Docs Size less is better ;
  • Docs Examined less is better ;
  • nReturned=totalDocsExamined=totalKeysExamined ;
  • SORT in index :sort 也是命中索引,否则,需要拿到数据后,再执行一遍排序;
  • Limit Array elements : 限定数组返回的条数,数组也不应该太多数据,否则schema 设计不合理。

彩蛋

文末,还有最开头1个问题没回答:如果我的索引改加的都加了,还不够快,怎么办?
留个悬念,之后再写一篇。

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