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我们知道mongodb是一种文本数据库，它和mysql数据库一样，都存在索引这一概念。给字段添加索引，可以加快数据查询的速度。当然了，在数据量足够大的情况下，加索引和不加索引的差距是很明显的。但是加了索引之后，在保存方面大概是会很浪费时间的。
1、单一索引
mongodb中使用以下命令来给字段添加索引。现在先简单的介绍一下，项目中用到的两种单一索引、集合索引。首先在mongodb中添加一个users集合：
> for(var i=0;i<100000;i++){ db.users.insert({username:'user'+i}) }
WriteResult({ "nInserted" : 1 })查看该集合中存在哪些索引：
> db.users.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.users"
}
]我们先来看一下查询user99999需要用多长的时间：我们可以看到"executionTimeMillis" : 50，也就是查询这一条记录，一共需要遍历"totalDocsExamined" : 100200，消耗了50毫秒。
> db.users.find({"username":"user99999"}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.users",
"indexFilterSet" : false,
"parsedQuery" : {
"username" : {
"$eq" : "user99999"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"username" : {
"$eq" : "user99999"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 50,
"totalKeysExamined" : 0,
"totalDocsExamined" : 100200,
"executionStages" : {
"stage" : "COLLSCAN",
"filter" : {
"username" : {
"$eq" : "user99999"
}
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 46,
"works" : 100202,
"advanced" : 1,
"needTime" : 100200,
"needYield" : 0,
"saveState" : 783,
"restoreState" : 783,
"isEOF" : 1,
"invalidates" : 0,
"direction" : "forward",
"docsExamined" : 100200
}
},
"serverInfo" : {
"host" : "guolujiedeMacBook-Pro.local",
"port" : 27017,
"version" : "4.0.2",
"gitVersion" : "fc1573ba18aee42f97a3bb13b67af7d837826b47"
},
"ok" : 1
}我们现在给该集合的username加上索引，现在有两个索引：
> db.users.createIndex({"username":1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
> db.users.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.users"
},
{
"v" : 2,
"key" : {
"username" : 1
},
"name" : "username_1",
"ns" : "test.users"
}
]我们再来查询一下加上索引之后需要消耗的时间：竟然只用了2毫秒，只需要查询一条记录。是不是效率提高了很多。
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 2,
"totalKeysExamined" : 1,
"totalDocsExamined" : 1,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 1,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"username" : 1
},
"indexName" : "username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"username" : [
"[\"user99999\", \"user99999\"]"
]
},
"keysExamined" : 1,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},2、联合索引
联合索引的添加方法如下：
> db.students.createIndex({"stuId":1,"name":1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}
> db.students.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.students"
},
{
"v" : 2,
"key" : {
"stuId" : 1,
"name" : 1
},
"name" : "stuId_1_name_1",
"ns" : "test.students"
}
]删除指定索引的方法如下：
> db.students.dropIndex("stuId_1_name_1")
{ "nIndexesWas" : 2, "ok" : 1 }删除所有索引的方法如下：
> db.students.dropIndexes()
{
"nIndexesWas" : 1,
"msg" : "non-_id indexes dropped for collection",
"ok" : 1
}