查看查询计划、查询计划的执行统计的方法：db.collection.explain()、cursor.explain()、explain
explain输出默认只输出queryPlanner、serverInfo部分，如果需要输出executionStats，则可以指定explain(allPlansExecution)或者explain("executionStats")
queryPlanner：执行计划相关描述
.COLLSCAN for a collection scan
.IXSCAN for scanning index keys
.FETCH for retrieving documents
.SHARD_MERGE for merging results from shards
.IDHACK 针对_id进行查询
.SHARDING_FILTER 通过mongos对分片数据进行查询
.COUNT 利用db.coll.explain().count()之类进行count运算
.COUNTSCAN count不使用用Index进行count时的stage返回
.COUNT_SCAN count使用了Index进行count时的stage返回
.SUBPLA 未使用到索引的$or查询的stage返回
.TEXT 使用全文索引进行查询时候的stage返回
.PROJECTION 限定返回字段时候stage的返回
.AND_SORTED 表示Index Intersection
.SORT 表示在内存中排序
.EOF 表示结果不存在
If MongoDB can use an index scan to obtain the requested sort order, the result will not include a SORT stage. Otherwise, if MongoDB cannot use the index to sort, the explain result will include a SORT stage.
样例
db.common_log_new.explain("allPlansExecution").find({ctime:{$lte:1529474045}}).limit(10)
{
"queryPlanner" : { --queryPlanner模式下并不会去真正进行query语句查询，而是针对query语句进行执行计划分析并选出winning plan。
"plannerVersion" : 1,
"namespace" : "dresslily.common_log_new",
"indexFilterSet" : false, --指定mongodb是否应用索引过滤
"parsedQuery" : {
"ctime" : {
"$lte" : 1529474045 --具体的查询语句
}
},
"winningPlan" : { --查询优化器选择的最优执行计划
"stage" : "LIMIT", --将检索出来的文档做limit处理
"limitAmount" : 10,
"inputStage" : { --描述子stage的文档，提供文档或者索引给他的父级stage，该值表示是否父级stage只有一个子节点
"stage" : "FETCH", --通过返回的index位置去检索具体的文档
"inputStage" : {
"stage" : "IXSCAN", --索引扫描，这个地方可以判断该查询是通过索引还是全集合扫描数据
"keyPattern" : { --所扫描的Index内容
"ctime" : 1
},
"indexName" : "idx_ctime", --索引名称
"isMultiKey" : false, --是否为多键索引
"isUnique" : false, --是否为唯一索引
"isSparse" : false, --是否为稀疏索引
"isPartial" : false, --是否为部分索引
"indexVersion" : 1,
"direction" : "forward", --query的查询顺序，如果用了.sort({w:-1})将显示backward。
"indexBounds" : { --winningplan所扫描的索引范围
"ctime" : [
"[-inf.0, 1529474045.0]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : { --query语句查询的过程
"executionSuccess" : true,
"nReturned" : 10, --查询的返回条数
"executionTimeMillis" : 53, --整体执行时长
"totalKeysExamined" : 10, --索引扫描条目数
"totalDocsExamined" : 10, --document扫描条目数，如果索引可以覆盖查询，那么这个地方应该是0，比较好的结果是totalDocsExamined=totalKeysExamined
"executionStages" : {
"stage" : "LIMIT", --与queryPlanner.winningPlan.stage部分对应，影响totalKeysExamined与totalDocsExamined
"nReturned" : 10,
"executionTimeMillisEstimate" : 0,
"works" : 11,
"advanced" : 10,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"limitAmount" : 10,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 10,
"executionTimeMillisEstimate" : 0,
"works" : 10,
"advanced" : 10,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 10,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 10,
"executionTimeMillisEstimate" : 0,
"works" : 10,
"advanced" : 10,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"ctime" : 1
},
"indexName" : "idx_ctime",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"ctime" : [
"[-inf.0, 1529474045.0]"
]
},
"keysExamined" : 10,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"allPlansExecution" : [ ]
},
"serverInfo" : {
"host" : "dresslilypcmongodb01.globalerow.com",
"port" : 27017,
"version" : "3.2.15",
"gitVersion" : "e11e3c1b9c9ce3f7b4a79493e16f5e4504e01140"
},
"ok" : 1}
