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。
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
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