Explain and Index
explain有三种模式，分别是：queryPlanner、executionStats、allPlansExecution。现实开发中，常用executionStats模式，本例分析这种模式。
> db.createCollection("person")
{ "ok" : 1 }> for(var i=0;i<2000000;i++) {
... db.person.insert({"name":"ryan"+i,"age":i})
... }
WriteResult({ "nInserted" : 1 })> db.person.find({"age":{"$lte":2000}}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.person",
"indexFilterSet" : false,
"parsedQuery" : {
"age" : {
"$lte" : 2000
}
},
"winningPlan" : {
"stage" : "COLLSCAN", "COLLSCAN"表示全表扫描
"filter" : {
"age" : {
"$lte" : 2000
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 2001, 表示查询返回的条目数
"executionTimeMillis" : 741, 表示执行语句的执行时间
"totalKeysExamined" : 0,
"totalDocsExamined" : 2000000, 表示 文档扫描的条目数
"executionStages" : {
"stage" : "COLLSCAN",
"filter" : {
"age" : {
"$lte" : 2000
}
},
"nReturned" : 2001,
"executionTimeMillisEstimate" : 530, 表示整体的查询时间
"works" : 2000002,
"advanced" : 2001,
"needTime" : 1998000,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"direction" : "forward",
"docsExamined" : 2000000
}
},
"serverInfo" : {
"host" : "meteor.yeecall.com",
"port" : 27027,
"version" : "3.2.8",
"gitVersion" : "ed70e33130c977bda0024c125b56d159573dbaf0"
},
"ok" : 1
}> db.person.ensureIndex({age:1}) 创建索引，“1”：表示按照age进行升序，“-1”：表示按照age进行降序
创建索引时ensureIndex({field:1},{name:"indexName"}) 可以指定索引的名称
也可以后台创建索引： db.collection.ensureIndex({name:-1},{background:true})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}> db.person.find({"age":{"$lte":2001}}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.person", 返回所查询的表
"indexFilterSet" : false, 针对该query是否有indexfilter
"parsedQuery" : {
"age" : {
"$lte" : 2001
}
},
"winningPlan" : { 查询优化器针对该query所返回的最优执行计划的详细内容
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1
},
"indexName" : "age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[-inf.0, 2001.0]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 2002, 表示本次查询返回的条目数
"executionTimeMillis" : 28, 表示本次query整体查询的时间
"totalKeysExamined" : 2002, 表示索引扫描的条目数
"totalDocsExamined" : 2002, 表示文档扫描的条目数
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 2002,
"executionTimeMillisEstimate" : 10, 表示本次查询根据index检索document获得结果数据的时间
"works" : 2003,
"advanced" : 2002,
"needTime" : 0,
"needYield" : 0,
"saveState" : 15,
"restoreState" : 15,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 2002,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 2002,
"executionTimeMillisEstimate" : 0, 表示该查询扫描2002行index所用的时间
"works" : 2003,
"advanced" : 2002,
"needTime" : 0,
"needYield" : 0,
"saveState" : 15,
"restoreState" : 15,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"age" : 1
},
"indexName" : "age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[-inf.0, 2001.0]"
]
},
"keysExamined" : 2002,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"serverInfo" : {
"host" : "meteor.yeecall.com",
"port" : 27027,
"version" : "3.2.8",
"gitVersion" : "ed70e33130c977bda0024c125b56d159573dbaf0"
},
"ok" : 1
}>
影响 totalKeysExamined 和 totalDocsExamined 的是 stage 类型。
类型列举如下：
COLLSCAN：全表扫描
IXSCAN：索引扫描
FETCH：根据索引去检索指定document
SHARD_MERGE：将各个分片返回数据进行merge
SORT：表明在内存中进行了排序
LIMIT：使用limit限制返回数
SKIP：使用skip进行跳过
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
