复合索引 <接上>
> 删除之前的collection，重新建立，如下所示：
> db.person.drop()
true
> for(var i=0;i<2000000;i++){
... db.person.insert({"name":"meteor"+i%1000,"age":20+i%10});
... }
WriteResult({ "nInserted" : 1 })
>
> db.person.ensureIndex({"age":1}) 创建单一索引
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}> db.person.ensureIndex({"name":1,"age":1}) 创建复合索引 {"name":1,"age":1}
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 2,
"numIndexesAfter" : 3,
"ok" : 1
}> db.person.ensureIndex({"age":1,"name":1}) 创建复合索引{"age":1,"name":1}
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 3,
"numIndexesAfter" : 4,
"ok" : 1
}> 查找时指定多个条件，使用hint强制指定使用单一索引速度比较慢，如下所示:
> db.person.find({"age":{"$gte":20,"$lte":30},"name":"meteor1"}).hint({"age":1}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.person",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"name" : {
"$eq" : "meteor1"
}
},
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 20
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"name" : {
"$eq" : "meteor1"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1
},
"indexName" : "age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[20.0, 30.0]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 2000,
"executionTimeMillis" : 2621,
"totalKeysExamined" : 2000000,
"totalDocsExamined" : 2000000,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"name" : {
"$eq" : "meteor1"
}
},
"nReturned" : 2000,
"executionTimeMillisEstimate" : 2050,
"works" : 2000001,
"advanced" : 2000,
"needTime" : 1998000,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 2000000,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 2000000,
"executionTimeMillisEstimate" : 640,
"works" : 2000001,
"advanced" : 2000000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"age" : 1
},
"indexName" : "age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[20.0, 30.0]"
]
},
"keysExamined" : 2000000,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"serverInfo" : {
"host" : "meteor.yeecall.com",
"port" : 27027,
"version" : "3.2.8",
"gitVersion" : "ed70e33130c977bda0024c125b56d159573dbaf0"
},
"ok" : 1
}> 查找数据时指定多个条件，使用hint强制指定使用{"age":1,"name":1}复合索引，速度比较快，如下所示：
> db.person.find({"age":{"$gte":20,"$lte":30},"name":"meteor1"}).hint({"age":1,"name":1}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.person",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"name" : {
"$eq" : "meteor1"
}
},
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 20
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1,
"name" : 1
},
"indexName" : "age_1_name_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[20.0, 30.0]"
],
"name" : [
"[\"meteor1\", \"meteor1\"]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 2000,
"executionTimeMillis" : 15,
"totalKeysExamined" : 2010,
"totalDocsExamined" : 2000,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 2000,
"executionTimeMillisEstimate" : 10,
"works" : 2011,
"advanced" : 2000,
"needTime" : 10,
"needYield" : 0,
"saveState" : 15,
"restoreState" : 15,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 2000,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 2000,
"executionTimeMillisEstimate" : 10,
"works" : 2011,
"advanced" : 2000,
"needTime" : 10,
"needYield" : 0,
"saveState" : 15,
"restoreState" : 15,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"age" : 1,
"name" : 1
},
"indexName" : "age_1_name_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[20.0, 30.0]"
],
"name" : [
"[\"meteor1\", \"meteor1\"]"
]
},
"keysExamined" : 2010,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"serverInfo" : {
"host" : "meteor.yeecall.com",
"port" : 27027,
"version" : "3.2.8",
"gitVersion" : "ed70e33130c977bda0024c125b56d159573dbaf0"
},
"ok" : 1
}=================================================================================
查询结束再次排序(按name排序)，并使用limit截取其中一部分，使用hint强制指定使用{"age":1,"name":1}索引时速度较慢，如下所示：
> db.person.find({"age":{"$gte":20,"$lte":30}}).sort({"name":1}).limit(100).hint({"age":1,"name":1}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.person",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 20
}
}
]
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"name" : 1
},
"limitAmount" : 100,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1,
"name" : 1
},
"indexName" : "age_1_name_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[20.0, 30.0]"
],
"name" : [
"[MinKey, MaxKey]"
]
}
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 100,
"executionTimeMillis" : 6991,
"totalKeysExamined" : 2000000,
"totalDocsExamined" : 2000000,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 100,
"executionTimeMillisEstimate" : 5980,
"works" : 2000103,
"advanced" : 100,
"needTime" : 2000002,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"sortPattern" : {
"name" : 1
},
"memUsage" : 6100,
"memLimit" : 33554432,
"limitAmount" : 100,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"nReturned" : 0,
"executionTimeMillisEstimate" : 5680,
"works" : 2000002,
"advanced" : 0,
"needTime" : 1,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 2000000,
"executionTimeMillisEstimate" : 4870,
"works" : 2000001,
"advanced" : 2000000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 2000000,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 2000000,
"executionTimeMillisEstimate" : 2400,
"works" : 2000001,
"advanced" : 2000000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 15625,
"restoreState" : 15625,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"age" : 1,
"name" : 1
},
"indexName" : "age_1_name_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[20.0, 30.0]"
],
"name" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 2000000,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
}
},
"serverInfo" : {
"host" : "meteor.yeecall.com",
"port" : 27027,
"version" : "3.2.8",
"gitVersion" : "ed70e33130c977bda0024c125b56d159573dbaf0"
},
"ok" : 1
}
>查询结束再次排序(按name排序)，并使用limit截取其中一部分，使用hint强制指定使用{"name":1,"age":1}索引时速度较快，如下所示：
> db.person.find({"age":{"$gte":20,"$lte":30}}).sort({"name":1}).limit(100).hint({"name":1,"age":1}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.person",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 20
}
}
]
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 100,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 20
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"name" : 1,
"age" : 1
},
"indexName" : "name_1_age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"name" : [
"[MinKey, MaxKey]"
],
"age" : [
"[MinKey, MaxKey]"
]
}
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 100,
"executionTimeMillis" : 5,
"totalKeysExamined" : 100,
"totalDocsExamined" : 100,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 100,
"executionTimeMillisEstimate" : 0,
"works" : 101,
"advanced" : 100,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"limitAmount" : 100,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 20
}
}
]
},
"nReturned" : 100,
"executionTimeMillisEstimate" : 0,
"works" : 100,
"advanced" : 100,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 0,
"invalidates" : 0,
"docsExamined" : 100,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 100,
"executionTimeMillisEstimate" : 0,
"works" : 100,
"advanced" : 100,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 0,
"invalidates" : 0,
"keyPattern" : {
"name" : 1,
"age" : 1
},
"indexName" : "name_1_age_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"name" : [
"[MinKey, MaxKey]"
],
"age" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 100,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
},
"serverInfo" : {
"host" : "meteor.yeecall.com",
"port" : 27027,
"version" : "3.2.8",
"gitVersion" : "ed70e33130c977bda0024c125b56d159573dbaf0"
},
"ok" : 1
}> 如果按age排序，索引使用{"name":1,"age":1}速度非常慢；如果按age排序，索引使用{"age":1,"name":1}速度比较快
>分析：第一种索引，需要找到所有复合查询条件的值（依据索引，键和文档可以快速找到），但是找到后，需要对文档在内存中进行排序，这个步骤消耗了非常多的时间。第二种索引，效果非常好，因为不需要在内存中对大量数据进行排序。但是，MongoDB不得不扫描整个索引以便找到所有文档。因此，如果对查询结果的范围做了限制，那么MongoDB在几次匹配之后就可以不再扫描索引，在这种情况下，将排序键放在第一位是一个非常好的策略。
查看索引
> db.person.getIndexes()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.person"
},
{
"v" : 1,
"key" : {
"age" : 1
},
"name" : "age_1",
"ns" : "test.person"
},
{
"v" : 1,
"key" : {
"name" : 1,
"age" : 1
},
"name" : "name_1_age_1",
"ns" : "test.person"
},
{
"v" : 1,
"key" : {
"age" : 1,
"name" : 1
},
"name" : "age_1_name_1",
"ns" : "test.person"
}
]> db.person.dropIndex("name_1_age_1") 删除索引
{ "nIndexesWas" : 4, "ok" : 1 }> db.person.dropIndex("age_1_name_1")
{ "nIndexesWas" : 3, "ok" : 1 }> db.person.dropIndex("age_1")
{ "nIndexesWas" : 2, "ok" : 1 }> db.runCommand({dropIndexes:"person",index:"*"}) 删除索引的另一种方法
{
"nIndexesWas" : 1,
"msg" : "non-_id indexes dropped for collection",
"ok" : 1
}> db.person.ensureIndex({"name":1,"age":1},{"unique":true}) 创建唯一索引 （本例没有成功，因为集合中有重复内容)
{
"ok" : 0,
"errmsg" : "E11000 duplicate key error collection: test.person index: name_1_age_1 dup key: { : \"meteor0\", : 20.0 }",
"code" : 11000
}MongoDB 索引限制
额外开销
每个索引占据一定的存储空间，在进行插入，更新和删除操作时也需要对索引进行操作。所以，如果你很少对集合进行读取操作，建议不使用索引。
内存(RAM)使用
由于索引是存储在内存(RAM)中,应该确保该索引的大小不超过内存的限制。(如上文中提示sort排序后，如果没有limit字段系统会提示错误，因为索引大小超过了内存的限制)
如果索引的大小大于内存的限制，MongoDB会删除一些索引，这将导致性能下降。
查询限制
索引不能被以下的查询使用：正则表达式及非操作符，如 $nin, $not, 等；算术运算符，如 $mod, 等；$where 子句
所以，检测语句是否使用索引是一个好的习惯，可以用explain来查看。
索引键限制
从2.6版本开始，如果现有的索引字段的值超过索引键的限制，MongoDB中不会创建索引。
插入文档超过索引键限制
如果文档的索引字段值超过了索引键的限制，MongoDB不会将任何文档转换成索引的集合。与mongorestore和mongoimport工具类似。
最大范围
集合中索引不能超过64个；索引名的长度不能超过125个字符
一个复合索引最多可以有31个字段
