在集合上执行查询分心方法 db.collection.explain()，会返回查询相关的统计数据，通过对统计数据的分析，可以观察查询语句是否执行了期望的计划，还可以判断查询性能是否高效、查询的耗时情况等。
1.语法
db.collection.find(过滤条件).explain(queryPlanner|executionStats|allPlansExecution)
这里可以看到explain后面有三个参数，其中queryPlanner是默认参数。
2.queryPlanner
| 参数 | 含义 |
| plannerVersion | 查询版本 |
| namespace | 要查询的集合(该值返回的是该query所查询的表) 数据库.集合 |
| indexFilterSet | 针对该query是否有indexFilter（索引过滤） |
| parsedQuery | 查询条件 |
| winningPlan | 被选中的执行计划 |
| winningPlan.stage |
被选中执行计划的stage(查询方式)，常见的有：collscan(全表扫描)、ixscan(索引扫描)、FETCH(根据索引去检索文档)、 shard_merge(合并分片结果)、idhack(针对_id进行查询等) |
| winningPlan.inputStage | 用来描述子stage，并且为其父stage提供文档和索引关键字 |
| winningPlan.stage的childstage | 如果此处是ixscan，表示进行的是index scanning |
| winningPlan.keyPattern | 所扫描的index内容 |
| winningPlan.indexName | winning plan 所选用的index |
| winningPlan.isMultiKey | 是否是Multikey，此处返回的是false，如果索引建立在arry上，此处将是true |
| winningPlan.direction | 此query的查询顺序，此处是forward，如果用了sort({字段:-1}) 将显示backward |
| filter | 过滤条件 |
| winningPlain.indexBounds | winningplan所扫描的索引范围，如果没有制定返回就是[maxkey,minkey]，这主要是直接定位到mongodb的chunk中查找数据，加快数据读取 |
| rejectedPlans | 被拒绝的执行计划的详细返回，其中具体信息与winningPlan的返回中意义相同 |
| serverinfo | Mongodb服务器的详细信息 |
示例：
> db.student.find({"name":'zl'}).explain() { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "test.student", "indexFilterSet" : false, "parsedQuery" : { "name" : { "$eq" : "zl" } }, "queryHash" : "01AEE5EC", "planCacheKey" : "0BE5F32C", "winningPlan" : { "stage" : "FETCH", //表示根据索引去检索文档 "inputStage" : { "stage" : "IXSCAN", //这里用到了索引扫描 "keyPattern" : { "name" : 1 //索引所在的字段 }, "indexName" : "name_1", //索引名字 "isMultiKey" : false, "multiKeyPaths" : { "name" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "name" : [ "[\"zl\", \"zl\"]" ] } } }, "rejectedPlans" : [ { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "name" : 1, "age" : 1 }, "indexName" : "name_1_age_1", "isMultiKey" : false, "multiKeyPaths" : { "name" : [ ], "age" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "name" : [ "[\"zl\", \"zl\"]" ], "age" : [ "[MinKey, MaxKey]" ] } } } ] }, "serverInfo" : { "host" : "mongodb", "port" : 27019, "version" : "4.4.12", "gitVersion" : "51475a8c4d9856eb1461137e7539a0a763cc85dc" }, "ok" : 1 }
2.executionStats：executionStats会返回执行计划的一些统计信息
| 参数 | 含义 |
| executionSuccess | 是否执行成功 |
| nRetured | 返回的文档数 |
| executionTimeMillis | 执行耗时 |
| totalKeysExamined | 索引扫描次数 |
| totalDocsExamined | 文档扫描次数 |
| stage | 扫描方式，具体可选值与上下的相同 |
| nRetured | 查询document获得数据的时间 |
| executionTimeMillsEstimate | 检索document获得数据的时间 |
| inputStage.executionTimeMillisEstimate | 该查询扫描文档index所用时间 |
| works | 工作单元数，一个查询会分解成小的工作单元 |
| advanced | 优先返回的结果数 |
| docsExamined | 文档检查数目，与totalDocsExamined一致，检查了总共的document个数，从而返回上面的nReturned数量 |
示例
> db.student.find({"name":'zl'}).explain("executionStats") { "queryPlanner" : { "plannerVersion" : 1, "namespace" : "test.student", "indexFilterSet" : false, "parsedQuery" : { "name" : { "$eq" : "zl" } }, "winningPlan" : { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "name" : 1 }, "indexName" : "name_1", "isMultiKey" : false, "multiKeyPaths" : { "name" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "name" : [ "[\"zl\", \"zl\"]" ] } } }, "rejectedPlans" : [ { "stage" : "FETCH", "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "name" : 1, "age" : 1 }, "indexName" : "name_1_age_1", "isMultiKey" : false, "multiKeyPaths" : { "name" : [ ], "age" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "name" : [ "[\"zl\", \"zl\"]" ], "age" : [ "[MinKey, MaxKey]" ] } } } ] }, "executionStats" : { "executionSuccess" : true, "nReturned" : 0, "executionTimeMillis" : 0, "totalKeysExamined" : 0, "totalDocsExamined" : 0, "executionStages" : { "stage" : "FETCH", "nReturned" : 0, "executionTimeMillisEstimate" : 0, "works" : 2, "advanced" : 0, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "docsExamined" : 0, "alreadyHasObj" : 0, "inputStage" : { "stage" : "IXSCAN", "nReturned" : 0, "executionTimeMillisEstimate" : 0, "works" : 1, "advanced" : 0, "needTime" : 0, "needYield" : 0, "saveState" : 0, "restoreState" : 0, "isEOF" : 1, "keyPattern" : { "name" : 1 }, "indexName" : "name_1", "isMultiKey" : false, "multiKeyPaths" : { "name" : [ ] }, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 2, "direction" : "forward", "indexBounds" : { "name" : [ "[\"zl\", \"zl\"]" ] }, "keysExamined" : 0, "seeks" : 1, "dupsTested" : 0, "dupsDropped" : 0 } } }, "serverInfo" : { "host" : "mongodb", "port" : 27019, "version" : "4.4.12", "gitVersion" : "51475a8c4d9856eb1461137e7539a0a763cc85dc" }, "ok" : 1 }
3.allPlansExecution
allPlansExecution用来获取所有执行计划，结果参数基本上与前面大概相同