前言
本文主要給大家介紹了關于MongoDB中索引和explain使用的相關內容，分享出來供大家參考學習，下面話不多說了，來一起看看詳細的介紹：
mongodb 索引使用
作用
創建索引
db.collection.createIndex(keys, options)
keys
options
options 創建索引的選項。
| 參數 | 類型 | 描述 |
|---|---|---|
| background | boolean | 創建索引在后臺運行，不會阻止其他對數據庫操作 |
| unique | boolean | 創建唯一索引，文檔的值不會重復 |
| name | string | 索引名稱，默認是：字段名_排序類型 開始排序 |
| sparse | boolean | 過濾掉null，不存在的字段 |
查看索引
db.collection.getIndexes()
{ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.userdatas" }, { "v" : 1, "key" : { "name" : 1 //索引字段 }, "name" : "name_1", //索引名稱 "ns" : "leyue.userdatas" }刪除索引
db.collection.dropIndex(index) 刪除指定的索引。
db.collection.dropIndexes() 刪除除了_id 以外的所有索引。
創建/查看/刪除 示例
查看數據
db.userdatas.find(){ "_id" : ObjectId("597f357a09c84cf58880e412"), "name" : "u3", "age" : 32 }{ "_id" : ObjectId("597f357a09c84cf58880e411"), "name" : "u4", "age" : 30, "score" : [ 7, 4, 2, 0 ] }{ "_id" : ObjectId("597fcc0f411f2b2fd30d0b3f"), "age" : 20, "score" : [ 7, 4, 2, 0, 10, 9, 8, 7 ], "name" : "lihao" }{ "_id" : ObjectId("597f357a09c84cf58880e413"), "name" : "u2", "age" : 33, "wendang" : { "yw" : 80, "xw" : 90 } }{ "_id" : ObjectId("5983f5c88eec53fbcd56a7ca"), "date" : ISODate("2017-08-04T04:19:20.693Z") }{ "_id" : ObjectId("597f357a09c84cf58880e40e"), "name" : "u1", "age" : 26, "address" : "中國碭山" }{ "_id" : ObjectId("597f357a09c84cf58880e40f"), "name" : "u1", "age" : 37, "score" : [ 10, 203, 12, 43, 56, 22 ] }{ "_id" : ObjectId("597f357a09c84cf58880e410"), "name" : "u5", "age" : 78, "address" : "china beijing chaoyang" }給字段name 創建索引
// 創建索引 db.userdatas.createIndex({"name":1}) { "createdCollectionAutomatically" : false, "numIndexesBefore" : 1, "numIndexesAfter" : 2, "ok" : 1 } // 查看索引 db.userdatas.getIndexes() [ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.userdatas" }, { "v" : 1, "key" : { "name" : 1 }, "name" : "name_1", "ns" : "leyue.userdatas" } ]給字段name 創建索引并命名為myindex
db.userdatas.createIndex({"name":1}) db.userdatas.createIndex({"name":1},{"name":"myindex"}) db.userdatas.getIndexes() [ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.userdatas" }, { "v" : 1, "key" : { "name" : 1 }, "name" : "myindex", "ns" : "leyue.userdatas" } ]給字段name 創建索引 創建的過程在后臺執行
當mongodb 集合里面的數據過大時 創建索引很耗時，可以在放在后臺運行。
db.userdatas.dropIndex("myindex") db.userdatas.createIndex({"name":1},{"name":"myindex","background":true})給age 字段創建唯一索引
db.userdatas.createIndex({"age":-1},{"name":"ageIndex","unique":true,"sparse":true}) db.userdatas.getIndexes()[ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.userdatas" }, { "v" : 1, "key" : { "name" : 1 }, "name" : "myindex", "ns" : "leyue.userdatas", "background" : true }, { "v" : 1, "unique" : true, "key" : { "age" : -1 }, "name" : "ageIndex", "ns" : "leyue.userdatas", "sparse" : true }]// 插入一個已存在的age db.userdatas.insert({ "name" : "u8", "age" : 32})WriteResult({ "nInserted" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate key error index: leyue.userdatas.$ageIndex dup key: { : 32.0 }" }})創建復合索引
db.userdatas.createIndex({"name":1,"age":-1}) db.userdatas.getIndexes()[ { "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.userdatas" }, { "v" : 1, "key" : { "name" : 1, "age" : -1 }, "name" : "name_1_age_-1", "ns" : "leyue.userdatas" }]所有的字段都存在集合 system.indexes 中
db.system.indexes.find(){ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.userdatas" }{ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.scores" }{ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.test" }{ "v" : 1, "key" : { "user" : 1, "name" : 1 }, "name" : "myindex", "ns" : "leyue.test" }{ "v" : 1, "key" : { "_id" : 1 }, "name" : "_id_", "ns" : "leyue.mycapped" }{ "v" : 1, "key" : { "user" : 1 }, "name" : "user_1", "ns" : "leyue.test" }{ "v" : 1, "key" : { "name" : 1 }, "name" : "myindex", "ns" : "leyue.userdatas" }索引總結
1:創建索引時,1表示按升序存儲,-1表示按降序存儲。
2:可以創建復合索引,如果想用到復合索引,必須在查詢條件中包含復合索引中的前N個索引列
3: 如果查詢條件中的鍵值順序和復合索引中的創建順序不一致的話,
MongoDB可以智能的幫助我們調整該順序,以便使復合索引可以為查詢所用。
4: 可以為內嵌文檔創建索引,其規則和普通文檔創建索引是一樣的。
5: 一次查詢中只能使用一個索引,$or特殊,可以在每個分支條件上使用一個索引。
6: $where,$exists不能使用索引,還有一些低效率的操作符,比如:$ne,$not,$nin等。
7: 設計多個字段的索引時,應該盡量將用于精確匹配的字段放在索引的前面。
explain 使用
語法
db.collection.explain().<method(...)>
explain() 可以設置參數 ：
示例
for(var i=0;i<100000;i++) { db.test.insert({"user":"user"+i});}沒有使用索引
db.test.explain("executionStats").find({"user":"user200000"}){ "queryPlanner" : { "plannerVersion" : 1, "namespace" : "leyue.test", "indexFilterSet" : false, "parsedQuery" : { "user" : { "$eq" : "user200000" } }, "winningPlan" : { "stage" : "COLLSCAN", "filter" : { "user" : { "$eq" : "user200000" } }, "direction" : "forward" }, "rejectedPlans" : [ ] }, "executionStats" : { "executionSuccess" : true, "nReturned" : 2, "executionTimeMillis" : 326, "totalKeysExamined" : 0, "totalDocsExamined" : 1006497, "executionStages" : { "stage" : "COLLSCAN", "filter" : { "user" : { "$eq" : "user200000" } }, "nReturned" : 2, "executionTimeMillisEstimate" : 270, "works" : 1006499, "advanced" : 2, "needTime" : 1006496, "needYield" : 0, "saveState" : 7863, "restoreState" : 7863, "isEOF" : 1, "invalidates" : 0, "direction" : "forward", "docsExamined" : 1006497 } }, "serverInfo" : { "host" : "lihaodeMacBook-Pro.local", "port" : 27017, "version" : "3.2.1", "gitVersion" : "a14d55980c2cdc565d4704a7e3ad37e4e535c1b2" }, "ok" : 1}executionStats.executionTimeMillis: query的整體查詢時間。executionStats.nReturned : 查詢返回的條目。executionStats.totalKeysExamined ： 索引掃描條目。executionStats.totalDocsExamined： 文檔掃描條目。executionTimeMillis = 326 query 執行時間
nReturned=2 返回兩條數據
totalKeysExamined=0 沒有用到索引
totalDocsExamined 全文檔掃描
理想狀態：
nReturned=totalKeysExamined & totalDocsExamined=0
Stage狀態分析
| stage | 描述 |
|---|---|
| COLLSCAN | 全表掃描 |
| IXSCAN | 掃描索引 |
| FETCH | 根據索引去檢索指定document |
| SHARD_MERGE | 將各個分片返回數據進行merge |
| SORT | 表明在內存中進行了排序 |
| LIMIT | 使用limit限制返回數 |
| SKIP | 使用skip進行跳過 |
| IDHACK | 針對_id進行查詢 |
| SHARDING_FILTER | 通過mongos對分片數據進行查詢 |
| COUNT | 利用db.coll.explain().count()之類進行count運算 |
| COUNTSCAN | count不使用Index進行count時的stage返回 |
| COUNT_SCAN | count使用了Index進行count時的stage返回 |
| SUBPLA | 未使用到索引的$or查詢的stage返回 |
| TEXT | 使用全文索引進行查詢時候的stage返回 |
| PROJECTION | 限定返回字段時候stage的返回 |
對于普通查詢，我希望看到stage的組合(查詢的時候盡可能用上索引)：
Fetch+IDHACK
Fetch+ixscan
Limit+（Fetch+ixscan）
PROJECTION+ixscan
SHARDING_FITER+ixscan
COUNT_SCAN
不希望看到包含如下的stage：
COLLSCAN(全表掃描),SORT(使用sort但是無index),不合理的SKIP,SUBPLA(未用到index的$or),COUNTSCAN(不使用index進行count)
使用索引
db.test.createIndex({"user":1},{"name":"myindex","background":true}) db.test.explain("executionStats").find({"user":"user200000"}){ "queryPlanner" : { "plannerVersion" : 1, "namespace" : "leyue.test", "indexFilterSet" : false, "parsedQuery" : { "user" : { "$eq" : "user200000" } }, "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "user" : 1 }, "indexName" : "myindex", "isMultiKey" : false, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 1, "direction" : "forward", "indexBounds" : { "user" : [ "[/"user200000/", /"user200000/"]" ] } } }, "rejectedPlans" : [ ] }, "executionStats" : { "executionSuccess" : true, "nReturned" : 2, "executionTimeMillis" : 0, "totalKeysExamined" : 2, "totalDocsExamined" : 2, "executionStages" : { "stage" : "FETCH", "nReturned" : 2, "executionTimeMillisEstimate" : 0, "works" : 3, "advanced" : 2, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "invalidates" : 0, "docsExamined" : 2, "alreadyHasObj" : 0, "inputStage" : { "stage" : "IXSCAN", "nReturned" : 2, "executionTimeMillisEstimate" : 0, "works" : 3, "advanced" : 2, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "invalidates" : 0, "keyPattern" : { "user" : 1 }, "indexName" : "myindex", "isMultiKey" : false, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 1, "direction" : "forward", "indexBounds" : { "user" : [ "[/"user200000/", /"user200000/"]" ] }, "keysExamined" : 2, "dupsTested" : 0, "dupsDropped" : 0, "seenInvalidated" : 0 } } }, "serverInfo" : { "host" : "lihaodeMacBook-Pro.local", "port" : 27017, "version" : "3.2.1", "gitVersion" : "a14d55980c2cdc565d4704a7e3ad37e4e535c1b2" }, "ok" : 1}executionTimeMillis: 0
totalKeysExamined: 2
totalDocsExamined:2
nReturned:2
stage:IXSCAN
使用索引和不使用差距很大，合理使用索引，一個集合適合做 4-5 個索引。
總結
以上就是這篇文章的全部內容了，希望本文的內容對大家的學習或者工作能帶來一定的幫助，如果有疑問大家可以留言交流，謝謝大家對武林網的支持。
相關文章
http://www.mongoing.com/eshu_explain3
https://docs.mongodb.com/v3.2/reference/explain-results/#queryplanner
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