我不明白为什么以下查询是如此缓慢，解释说明尽管存在索引，它还需要74+秒才能完成。
dev_id和_id都被编入索引，我可以向你保证，它似乎根本没用。
db.DeviceLoginLog.find({"dev_id": "xxx"}).skip(0).limit(10).sort({"_id": -1}).explain("executionStats");
任何人都可以帮我解释解释输出并告诉我如何加快速度？
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "example.DeviceLoginLog",
"indexFilterSet" : false,
"parsedQuery" : {
"dev_id" : {
"$eq" : "xxx"
}
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"_id" : -1
},
"limitAmount" : 10,
"inputStage" : {
"stage" : "KEEP_MUTATIONS",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"dev_id" : 1
},
"indexName" : "dev_id_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"dev_id" : [
"[\"xxx\", \"xxx\"]"
]
}
}
}
}
},
"rejectedPlans" : [
{
"stage" : "LIMIT",
"limitAmount" : 4,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"dev_id" : {
"$eq" : "xxx"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"direction" : "backward",
"indexBounds" : {
"_id" : [
"[MaxKey, MinKey]"
]
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 10,
"executionTimeMillis" : 74867,
"totalKeysExamined" : 9203,
"totalDocsExamined" : 9203,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 10,
"executionTimeMillisEstimate" : 49860,
"works" : 9537,
"advanced" : 10,
"needTime" : 9204,
"needFetch" : 321,
"saveState" : 1864,
"restoreState" : 1864,
"isEOF" : 1,
"invalidates" : 1064,
"sortPattern" : {
"_id" : -1
},
"memUsage" : 1890,
"memLimit" : 33554432,
"limitAmount" : 10,
"inputStage" : {
"stage" : "KEEP_MUTATIONS",
"nReturned" : 9203,
"executionTimeMillisEstimate" : 49820,
"works" : 9525,
"advanced" : 9203,
"needTime" : 0,
"needFetch" : 321,
"saveState" : 1864,
"restoreState" : 1864,
"isEOF" : 1,
"invalidates" : 1064,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 9203,
"executionTimeMillisEstimate" : 49820,
"works" : 9525,
"advanced" : 9203,
"needTime" : 0,
"needFetch" : 321,
"saveState" : 1864,
"restoreState" : 1864,
"isEOF" : 1,
"invalidates" : 1064,
"docsExamined" : 9203,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 9203,
"executionTimeMillisEstimate" : 10,
"works" : 9204,
"advanced" : 9203,
"needTime" : 0,
"needFetch" : 0,
"saveState" : 1864,
"restoreState" : 1864,
"isEOF" : 1,
"invalidates" : 1064,
"keyPattern" : {
"dev_id" : 1
},
"indexName" : "dev_id_1",
"isMultiKey" : false,
"direction" : "forward",
"indexBounds" : {
"dev_id" : [
"[\"xxx\", \"xxx\"]"
]
},
"keysExamined" : 9203,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0,
"matchTested" : 0
}
}
}
}
},
"serverInfo" : {
"host" : "iZ231ear7c9Z",
"port" : 27017,
"version" : "3.0.3",
"gitVersion" : "b40106b36eecd1b4407eb1ad1af6bc60593c6105"
},
"ok" : 1
}
提前致谢！
该查询使用索引{dev_id: 1}查找，然后必须将匹配的文档提取到内存中，以便在不使用索引的情况下进行排序。创建一个复合索引，因为{dev_id: 1, _id: -1}将提高性能。
"totalKeysExamined" : 9203,
"totalDocsExamined" : 9203,
这表明查询必须检查每个单独的文档。您没有在_id字段上使用索引。另外，根据您要返回的字段，请考虑创建允许覆盖查询的索引。
覆盖查询是可以使用索引完全满足的查询，而不必检查任何文档。当以下两个条件适用时，索引将涵盖查询：
- 查询中的所有字段都是索引的一部分，并且
- 结果中返回的所有字段都在同一索引中。