MongoDB 复合索引的试验

根据典型碰到的场景，来做几个实验：
这里创建了个loans collection。简化只有100条数据。这个是借贷的表有 _id, userId, status(借贷状态), amount(金额).

看完 这个实验后， 你会明白了 {userId:1, status:1}, vs {status:1,userId:1} 的差别

PS：这个case 里面其实status 区分度不高，不应该建立的，这里只是作为实例展示。

总结：

• 注意使用上 使用频率上 区分高的/常用的在前面
• 如果需要减少索引以节省memory/提高修改数据的性能的话，可以保留区分度高，常用的，去除区分度不高，不常用的索引。

实验如下：

db.loans.count()
100

db.loans.find({ "userId" : "59e022d33f239800129c61c7", "status" : "repayed", }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"$and" : [
{
"status" : {
"$eq" : "repayed"
}
},
{
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
}
]
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"$and" : [
{
"status" : {
"$eq" : "repayed"
}
},
{
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
}
]
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

注意上面 COLLSCAN 全表扫描了。因为没有索引。
next 我们分别建立几个索引

step 1 先建立 {userId:1, status:1}

db.loans.createIndex({userId:1, status:1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}

db.loans.find({ "userId" : "59e022d33f239800129c61c7", "status" : "repayed", }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"$and" : [
{
"status" : {
"$eq" : "repayed"
}
},
{
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
],
"status" : [
"["repayed", "repayed"]"
]
}
}
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

如愿命中 {userId:1, status:1} 作为 winning plan

step2 再建立个典型的索引 userId

db.loans.createIndex({userId:1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 2,
"numIndexesAfter" : 3,
"ok" : 1
}

db.loans.find({ "userId" : "59e022d33f239800129c61c7", "status" : "repayed", }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"$and" : [
{
"status" : {
"$eq" : "repayed"
}
},
{
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
],
"status" : [
"["repayed", "repayed"]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"filter" : {
"status" : {
"$eq" : "repayed"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1
},
"indexName" : "userId_1",
"multiKeyPaths" : {
"userId" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
]
}
}
}
]
},
"ok" : 1
}

留意到 DB 检测到 {userId:1, status:1} 为更优执行的方案

db.loans.find({ "userId" : "59e022d33f239800129c61c7" }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1
},
"indexName" : "userId_1",
"multiKeyPaths" : {
"userId" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
],
"status" : [
"[MinKey, MaxKey]"
]
}
}
}
]
},
"ok" : 1
}

留意到 DB 检测到 {userId:1} 为更优执行的方案，嗯~，如我们所料

db.loans.find({ "status" : "repayed" }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"status" : {
"$eq" : "repayed"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"status" : {
"$eq" : "repayed"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

***有趣的部分： status 不命中索引， 全表扫描 ***
接下来，我加了个sort

db.loans.find({ "userId" : "59e022d33f239800129c61c7" }).sort({status:1}).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
],
"status" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "SORT",
"sortPattern" : {
"status" : 1
},
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1
},
"indexName" : "userId_1",
"multiKeyPaths" : {
"userId" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
]
}
}
}
}
}
]
},
"ok" : 1
}

***有趣的部分： status 不命中索引 ***

db.loans.find({ "status" : "repayed","userId" : "59e022d33f239800129c61c7", }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"$and" : [
{
"status" : {
"$eq" : "repayed"
}
},
{
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
}
]
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
],
"status" : [
"["repayed", "repayed"]"
]
}
}
},
"rejectedPlans" : [
{
"stage" : "FETCH",
"filter" : {
"status" : {
"$eq" : "repayed"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1
},
"indexName" : "userId_1",
"multiKeyPaths" : {
"userId" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
]
}
}
}
]
},
"ok" : 1
}

命中索引， 跟 query 的各个字段顺序不相关，如我们猜测

有趣部分再来， 我们删掉索引{userId:1}

db.loans.dropIndex({"userId":1})
{ "nIndexesWas" : 3, "ok" : 1 }

db.loans.find({"userId" : "59e022d33f239800129c61c7", }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"["59e022d33f239800129c61c7", "59e022d33f239800129c61c7"]"
],
"status" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

DB 执行分析器觉得索引{userId:1, status:1} 能更优

没有命中复合索引 ，这个是因为status 不是 leading field

db.loans.find({ "status" : "repayed" }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"status" : {
"$eq" : "repayed"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"status" : {
"$eq" : "repayed"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

再换个角度sort 一遍， 与前面query & sort 互换 ，之前是

db.loans.find({userId:1}).sort({ "status" : "repayed" })
看看有啥不一样？

db.loans.find({ "status" : "repayed" }).sort({userId:1}).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"status" : {
"$eq" : "repayed"
}
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"status" : {
"$eq" : "repayed"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"userId" : 1,
"status" : 1
},
"indexName" : "userId_1_status_1",
"multiKeyPaths" : {
"userId" : [ ],
"status" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"userId" : [
"[MinKey, MaxKey]"
],
"status" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

如猜测，命中索引

再来玩1玩，确认下leading filed试验：

db.loans.dropIndex("userId_1_status_1")
{ "nIndexesWas" : 2, "ok" : 1 }

db.loans.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "id",
"ns" : "cashLoan.loans"
}
]

db.loans.createIndex({status:1, userId:1})
{
"createdCollectionAutomatically" : false,
"numIndexesBefore" : 1,
"numIndexesAfter" : 2,
"ok" : 1
}

db.loans.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "id",
"ns" : "cashLoan.loans"
},
{
"v" : 2,
"key" : {
"status" : 1,
"userId" : 1
},
"name" : "status_1_userId_1",
"ns" : "cashLoan.loans"
}
]

db.loans.find({ "status" : "repayed" }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"status" : {
"$eq" : "repayed"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"status" : 1,
"userId" : 1
},
"indexName" : "status_1_userId_1",
"multiKeyPaths" : {
"status" : [ ],
"userId" : [ ]
},
"direction" : "forward",
"indexBounds" : {
"status" : [
"["repayed", "repayed"]"
],
"userId" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

status_1_userId_1 有这个索引的前提，去查 leading fields -- status: xx 会中

db.loans.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "id",
"ns" : "cashLoan.loans"
},
{
"v" : 2,
"key" : {
"status" : 1,
"userId" : 1
},
"name" : "status_1_userId_1",
"ns" : "cashLoan.loans"
}
]

db.loans.find({"userId" : "59e022d33f239800129c61c7", }).explain()
{
"queryPlanner" : {
"namespace" : "cashLoan.loans",
"parsedQuery" : {
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"userId" : {
"$eq" : "59e022d33f239800129c61c7"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"ok" : 1
}

status_1_userId_1 有这个索引的前提，去查 非leading fields -- user_id: xx 没中，全表扫描

所以 注意使用上 使用频率上 区分高的/常用的， 应该使用于混合索引，在前面作为leading fields，