mongodb查詢語句效率分析
阿新 • • 發佈：2019-01-01
準備工作
- 安裝最新版的mongodb.
- 安裝免費的圖形客戶端Robo 3T
- 在資料庫local下建立一個名為test的collection
- 建立索引
db.test.createIndex( { a: -1, b:-1 }, {name:"ab"}) - 準備一些資料
{_id:1, a:1, b:2} {_id:2, a:1, b:2} {_id:3, a:1, b:3} {_id:4, a:1, b:3} {_id:5, a:2, b:2} {_id:6, a:2, b:2} {_id:7, a:2, b:3} {_id:8, a:2, b:3}
索引分析
db.test.find({a:1}).explain();
執行結果
{ "queryPlanner" : { "plannerVersion" : 1, "namespace" : "local.test", "indexFilterSet" : false, "parsedQuery" : { "a" : { "$eq" : 1.0 } }, "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "a" : -1.0, "b" : -1.0 }, "indexName" : "ab", "isMultiKey" : false, "multiKeyPaths" : { "a" : [], "b" : [] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "a" : [ "[1.0, 1.0]" ], "b" : [ "[MaxKey, MinKey]" ] } } }, "rejectedPlans" : [] }, "serverInfo" : {}, "ok" : 1.0 }
關鍵引數說明
| 欄位 | 說明 |
|---|---|
| queryPlanner.indexFilterSet | 是否啟用了index filter, mongodb使用index filter來選擇索引 |
| queryPlanner.winningPlan | 優化器最終選擇的plan |
| queryPlanner.winningPlan.inputStage | 查詢輸入的引數和索引 |
| queryPlanner.winningPlan.inputStage.stage | 所處的階段COLLSCAN: 掃描collection IXSCAN掃描索引FETCH讀取文件 |
| queryPlanner.winningPlan.inputStage.keyPattern | 索引的格式, 與建立索引時的格式一致 |
| queryPlanner.winningPlan.inputStage.indexBounds | 查詢條件的約束值. |
| queryPlanner.rejectedPlans | 被拒絕掉的候選plan |
執行過程分析
db.test.find({a:1}).explain("executionStats");
{
"queryPlanner" : { 略 },
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 4,
"executionTimeMillis" : 0,
"totalKeysExamined" : 4,
"totalDocsExamined" : 4,
"executionStages" : {
"stage" : "FETCH",
"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" : {
"a" : -1.0,
"b" : -1.0
},
"indexName" : "ab",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : [],
"b" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[1.0, 1.0]"
],
"b" : [
"[MaxKey, MinKey]"
]
},
"keysExamined" : 4,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"serverInfo" : {},
"ok" : 1.0
}
關鍵欄位說明
| 欄位 | 說明 |
|---|---|
| executionStats.totalKeysExamined | 遍歷索引的次數 |
| executionStats.totalDocsExamined | 遍歷文件的次數 |
| executionTimeMillisEstimate | 預測需要執行的時間 |
遍歷的次數越多, 遍歷肯定越慢. 本例中使用了索引, 有4條滿足條件的文件, 所有遍歷次數都是4.
不使用索引
db.test.find({b:2}).explain("executionStats");
{
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 4,
"executionTimeMillis" : 0,
"totalKeysExamined" : 0,
"totalDocsExamined" : 8,
},
}
db.test.find({b:2})無法使用索引{a:1, b:1}, 所以沒有遍歷索引, 然後遍歷了整個collection
只返回索引資料
db.test.find({a:2}, {_id:0, a:1, b:1}).explain("executionStats");
{
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 4,
"executionTimeMillis" : 0,
"totalKeysExamined" : 4,
"totalDocsExamined" : 0,
}
},
}
本例要mongodb只返回a, b兩次欄位, 而這兩個欄位剛好建立了索引, 不需要再讀取document, 所以沒有遍歷文件.
總結
mongodb中的索引與mysql的索引的行為非常相似