1.索引简介
(1).准备基础数据
for (i=0;i<1000000; i++){
db.users.insertOne({"i":i,username:"user"+i,age:Math.floor(Math.random()*120),created:new Date()})
}(2).然后随机查找一个用户
wang> db.users.find({"username": "user101"}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "wang.users",
"indexFilterSet" : false,
"parsedQuery" : {
"username" : {
"$eq" : "user101"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"username" : {
"$eq" : "user101"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 848,
"totalKeysExamined" : 0,
"totalDocsExamined" : 1000000,
"executionStages" : {
"stage" : "COLLSCAN",
"filter" : {
"username" : {
"$eq" : "user101"
}
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 106,
"works" : 1000002,
"advanced" : 1,
"needTime" : 1000000,
"needYield" : 0,
"saveState" : 1000,
"restoreState" : 1000,
"isEOF" : 1,
"direction" : "forward",
"docsExamined" : 1000000
}
},
"serverInfo" : {
"host" : "localhost.localdomain",
"port" : 27017,
"version" : "4.4.0",
"gitVersion" : "563487e100c4215e2dce98d0af2a6a5a2d67c5cf"
},
"ok" : 1
}
wang> 其中重点关注executionStats的以下参数
totalDocsExamined 已扫描的文档总数
executionTimeMillis 执行该操作所需的毫秒数
nReturned 返回的文档数
totalKeysExamined 显示0表示此查询未使用索引(3).创建索引
db.users.createIndex({"username" : 1})如果要查询索引的创建进度，可以使用以下命令
db.currentOp()此时再次进行上面的查询（关注标红部分，增加索引后性能提升很大）：
wang> db.users.find({"username": "user101"}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "wang.users",
"indexFilterSet" : false,
"parsedQuery" : {
"username" : {
"$eq" : "user101"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"username" : 1
},
"indexName" : "username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"username" : [
"[\"user101\", \"user101\"]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 57,
"totalKeysExamined" : 1,
"totalDocsExamined" : 1,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 1,
"executionTimeMillisEstimate" : 10,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"docsExamined" : 1,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1,
"executionTimeMillisEstimate" : 10,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"keyPattern" : {
"username" : 1
},
"indexName" : "username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"username" : [
"[\"user101\", \"user101\"]"
]
},
"keysExamined" : 1,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"serverInfo" : {
"host" : "localhost.localdomain",
"port" : 27017,
"version" : "4.4.0",
"gitVersion" : "563487e100c4215e2dce98d0af2a6a5a2d67c5cf"
},
"ok" : 1
}
wang> 2.复合索引（Compound Indexes）
复合索引就是建立在一个或多个字段上的索引
db.users.createIndex({"age" : 1, "username" : 1})mongodb对这个索引的使用方式取决于查询的类型，下面是三种主要的方式。
(1).方式一
db.users.find({age:21}).sort({username:-1})这个查询中第一个字段是等值查询，第二个字段已经在索引中了，查询结果已经是有序的了，所以逆序便利索引即可，查看执行计划
wang> db.users.find({age:21}).sort({username:-1}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "wang.users",
"indexFilterSet" : false,
"parsedQuery" : {
"age" : {
"$eq" : 21
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1,
"username" : 1
},
"indexName" : "age_1_username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"age" : [ ],
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "backward",
"indexBounds" : {
"age" : [
"[21.0, 21.0]"
],
"username" : [
"[MaxKey, MinKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 8336,
"executionTimeMillis" : 47,
"totalKeysExamined" : 8336,
"totalDocsExamined" : 8336,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 8336,
"executionTimeMillisEstimate" : 14,
"works" : 8337,
"advanced" : 8336,
"needTime" : 0,
"needYield" : 0,
"saveState" : 8,
"restoreState" : 8,
"isEOF" : 1,
"docsExamined" : 8336,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 8336,
"executionTimeMillisEstimate" : 0,
"works" : 8337,
"advanced" : 8336,
"needTime" : 0,
"needYield" : 0,
"saveState" : 8,
"restoreState" : 8,
"isEOF" : 1,
"keyPattern" : {
"age" : 1,
"username" : 1
},
"indexName" : "age_1_username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"age" : [ ],
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "backward",
"indexBounds" : {
"age" : [
"[21.0, 21.0]"
],
"username" : [
"[MaxKey, MinKey]"
]
},
"keysExamined" : 8336,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"serverInfo" : {
"host" : "localhost.localdomain",
"port" : 27017,
"version" : "4.4.0",
"gitVersion" : "563487e100c4215e2dce98d0af2a6a5a2d67c5cf"
},
"ok" : 1
}
wang> 这种查询是非常高效的，mongodb能够直接定位到正确的年龄，而且不需要对结果进行排序(因为只需要对数据进行逆序遍历就可以得到正确的顺序了)。
注意。排序的方向并不重要，mongodb可以在任意方向上对索引进行遍历。
(2).方式二
db.users.find({"age" : {"$gte" : 21, "$lte" : 30}})这是一个多值查询，用于查找匹配多个值的文档（在此情况下，年龄在21至30岁之间）。 这种特定类型的查询有时也称为范围查询。 MongoDB将使用索引中的第一个键age返回匹配的文档
wang> db.users.find({"age" : {"$gte" : 21, "$lte" : 30}}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "wang.users",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 21
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1,
"username" : 1
},
"indexName" : "age_1_username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"age" : [ ],
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[21.0, 30.0]"
],
"username" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 83522,
"executionTimeMillis" : 333,
"totalKeysExamined" : 83522,
"totalDocsExamined" : 83522,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 83522,
"executionTimeMillisEstimate" : 73,
"works" : 83523,
"advanced" : 83522,
"needTime" : 0,
"needYield" : 0,
"saveState" : 83,
"restoreState" : 83,
"isEOF" : 1,
"docsExamined" : 83522,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 83522,
"executionTimeMillisEstimate" : 32,
"works" : 83523,
"advanced" : 83522,
"needTime" : 0,
"needYield" : 0,
"saveState" : 83,
"restoreState" : 83,
"isEOF" : 1,
"keyPattern" : {
"age" : 1,
"username" : 1
},
"indexName" : "age_1_username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"age" : [ ],
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[21.0, 30.0]"
],
"username" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 83522,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"serverInfo" : {
"host" : "localhost.localdomain",
"port" : 27017,
"version" : "4.4.0",
"gitVersion" : "563487e100c4215e2dce98d0af2a6a5a2d67c5cf"
},
"ok" : 1
}
wang> (3).方式三
db.users.find({"age" : {"$gte" : 21, "$lte" :30}}).sort({"username" : 1})由于第一个字段使用了范围查询，所以第二哥字段无法使用索引，所以必须在内存中进行排序，因此这种查询不如第一个高效
wang> db.users.find({"age" : {"$gte" : 21, "$lte" :30}}).sort({"username" : 1}).explain("executionStats")
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "wang.users",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 21
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 21
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"username" : 1
},
"indexName" : "username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"username" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"inputStage" : {
"stage" : "SORT",
"sortPattern" : {
"username" : 1
},
"memLimit" : 104857600,
"type" : "default",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"age" : 1,
"username" : 1
},
"indexName" : "age_1_username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"age" : [ ],
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"age" : [
"[21.0, 30.0]"
],
"username" : [
"[MinKey, MaxKey]"
]
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 83522,
"executionTimeMillis" : 3292,
"totalKeysExamined" : 1000000,
"totalDocsExamined" : 1000000,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"age" : {
"$lte" : 30
}
},
{
"age" : {
"$gte" : 21
}
}
]
},
"nReturned" : 83522,
"executionTimeMillisEstimate" : 765,
"works" : 1000001,
"advanced" : 83522,
"needTime" : 916478,
"needYield" : 0,
"saveState" : 1001,
"restoreState" : 1001,
"isEOF" : 1,
"docsExamined" : 1000000,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1000000,
"executionTimeMillisEstimate" : 315,
"works" : 1000001,
"advanced" : 1000000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 1001,
"restoreState" : 1001,
"isEOF" : 1,
"keyPattern" : {
"username" : 1
},
"indexName" : "username_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"username" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"username" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 1000000,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"serverInfo" : {
"host" : "localhost.localdomain",
"port" : 27017,
"version" : "4.4.0",
"gitVersion" : "563487e100c4215e2dce98d0af2a6a5a2d67c5cf"
},
"ok" : 1
}
wang> 如果您的结果超过32MB结果MongoDB只会出错，拒绝对大量数据进行排序：
Error: error: {
"ok" : 0,
"errmsg" : "Executor error during find command: OperationFailed:
Sort operation used more than the maximum 33554432 bytes of RAM. Add
an index, or specify a smaller limit.",
"code" : 96,
"codeName" : "OperationFailed"
}如果使用 {"username" : 1, "age" : 1} 索引会得到很好的效果。
设计复合索引时：
等值查询的键应该首先出现。
用于排序的键应该出现在多值字段之前。
多值过滤器的键应该出现在最后。