背景
我有一组具有如下文档结构的用户：
{
"_id" : ObjectId("54e61137cca5d2ff0a8b4567"),
"login" : "test1",
"emails" : [
{
"email" : "[email protected]",
"is_primary" : true,
"_id" : ObjectId("57baf3e97323afb2688e639c")
},
{
"email" : "[email protected]",
"is_primary" : false,
"_id" : ObjectId("57baf3e97323afb2688e639d")
}
]
}
索引：
{
"v" : 1,
"key" : {
"login" : 1
},
"name" : "login_1",
"ns" : "mydb.users",
"background" : true
},
{
"v" : 1,
"key" : {
"emails.email" : 1
},
"name" : "emails.email_1",
"ns" : "mydb.users"
}
文档数量约为 700000 份
场景
为了解释通过登录搜索用户，我这样做：
rs0:PRIMARY> db.users.explain('executionStats').find({'login' : /test123123123/})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "mydb.users",
"indexFilterSet" : false,
"parsedQuery" : {
"login" : /test123123123/
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"login" : /test123123123/
},
"keyPattern" : {
"login" : 1
},
"indexName" : "login_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"login" : [
"[\"\", {})",
"[/test123123123/, /test123123123/]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 1040,
"totalKeysExamined" : 698993,
"totalDocsExamined" : 0,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 0,
"executionTimeMillisEstimate" : 930,
"works" : 698994,
"advanced" : 0,
"needTime" : 698993,
"needFetch" : 0,
"saveState" : 5460,
"restoreState" : 5460,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 0,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"login" : /test123123123/
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 920,
"works" : 698993,
"advanced" : 0,
"needTime" : 698993,
"needFetch" : 0,
"saveState" : 5460,
"restoreState" : 5460,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"login" : 1
},
"indexName" : "login_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"login" : [
"[\"\", {})",
"[/test123123123/, /test123123123/]"
]
},
"keysExamined" : 698993,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0
}
}
},
"serverInfo" : {
"host" : "myhost",
"port" : 27017,
"version" : "3.0.12",
"gitVersion" : "33934938e0e95d534cebbaff656cde916b9c3573"
},
"ok" : 1
}
你可以看到executionStats.executionStages.inputStage.nReturned是0，executionStats.totalDocsExamined是0。没关系，我猜没有像输入的登录文档。但如果我想通过电子邮件搜索用户，我接下来会做：
rs0:PRIMARY> db.users.explain('executionStats').find({'emails.email' : /test123123123/})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "mydb.users",
"indexFilterSet" : false,
"parsedQuery" : {
"emails.email" : /test123123123/
},
"winningPlan" : {
"stage" : "FETCH",
"filter" : {
"emails.email" : /test123123123/
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"emails.email" : 1
},
"indexName" : "emails.email_1",
"isMultiKey" : true,
"direction" : "forward",
"indexBounds" : {
"emails.email" : [
"[\"\", {})",
"[/test123123123/, /test123123123/]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 7666,
"totalKeysExamined" : 699016,
"totalDocsExamined" : 698993,
"executionStages" : {
"stage" : "FETCH",
"filter" : {
"emails.email" : /test123123123/
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 7355,
"works" : 699017,
"advanced" : 0,
"needTime" : 699016,
"needFetch" : 0,
"saveState" : 5462,
"restoreState" : 5462,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 698993,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 698993,
"executionTimeMillisEstimate" : 1630,
"works" : 699016,
"advanced" : 698993,
"needTime" : 23,
"needFetch" : 0,
"saveState" : 5462,
"restoreState" : 5462,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"emails.email" : 1
},
"indexName" : "emails.email_1",
"isMultiKey" : true,
"direction" : "forward",
"indexBounds" : {
"emails.email" : [
"[\"\", {})",
"[/test123123123/, /test123123123/]"
]
},
"keysExamined" : 699016,
"dupsTested" : 699016,
"dupsDropped" : 23,
"seenInvalidated" : 0,
"matchTested" : 0
}
}
},
"serverInfo" : {
"host" : "myhost",
"port" : 27017,
"version" : "3.0.12",
"gitVersion" : "33934938e0e95d534cebbaff656cde916b9c3573"
},
"ok" : 1
}
这里executionStats.executionStages.inputStage.nReturned（和executionStats.totalDocsExamined）等于698993（executionStats.nReturned是0，就像第一个查询一样）
问题
为什么当我在 ixscan 阶段使用多键索引 (users.user) 进行搜索时，会返回所有集合，而 fetch 阶段会出现所有集合。但是，如果我使用非多键索引（登录）搜索，ixscan 阶段会扫描预期值，并在获取阶段给出我想要的内容。
UPD：当我使用正则表达式而不是 /smth/ 时，而是 /^smth/ 然后通过 emails.email 字段扫描也返回 0 个元素。为什么多键和普通索引对于像 /smth/ 这样的正则表达式给出不同的结果？
因为是多键索引。 在这里解释
当查询过滤器指定整个数组的精确匹配时，MongoDB 可以使用多键索引来查找查询数组的第一个元素，但不能使用多键索引扫描来查找整个数组。相反，在使用多键索引查找查询数组的第一个元素后，MongoDB 会检索关联的文档并筛选其数组与查询中的数组匹配的文档。
当我使用正则表达式时，不是/smth/，而是/^smth/，然后扫描 通过 emails.email 字段也返回 0 个元素。为什么多键和 普通索引给我不同的正则表达式结果，例如 /smth/
这是因为 MongoDB 能够优化搜索，因为后一个查询包含一个锚点，特别是它应该以给定的字符串开头。因此它可以构造一个已索引的范围并很快找不到结果。否则，仅使用“包含”之类的搜索，无法完成此类优化（不会构建索引元素范围），因此 MongoDB 必须使用全集合扫描（返回所有元素）。请参阅https://stackoverflow.com/a/33219393/1527469