MongoDB即使使用索引也发现查询非常慢
[英]MongoDB find query very slow even with index
问题描述
I don't understand why the following query is so slow, the explain shows it will take 74+ seconds to finish, in spite of the presence of index. 
我不明白为什么下面的查询这么慢，尽管有索引，但说明却显示要花74+秒才能完成。
both dev_id and _id are indexed I can assure you, it seems just not helpful at all. 我可以向您保证， dev_id和_id都已建立索引，这似乎根本没有帮助。
db.DeviceLoginLog.find({"dev_id": "xxx"}).skip(0).limit(10).sort({"_id": -1}).explain("executionStats");
Can anyone help me to interpret explain output and advise me how to speed up? 谁能帮助我解释解释输出并建议我如何加快速度？
{
"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
}
Thanks in advance！ 提前致谢！
2 个解决方案
解决方案1
2 已采纳 2019-03-01 03:13:57
The query used index {dev_id: 1} to find and then had to fetch matched docs into memory to sort without using an index. 该查询使用索引{dev_id: 1}进行查找，然后必须将匹配的文档提取到内存中以进行排序而不使用索引。 Create a compound index as {dev_id: 1, _id: -1} will improve performance. 创建一个复合索引为{dev_id: 1, _id: -1}将提高性能。
See sort-and-non-prefix-subset-of-an-index . 请参阅索引的排序和非前缀子集 。
解决方案2
0 2019-03-01 03:16:22
"totalKeysExamined" : 9203,
"totalDocsExamined" : 9203,
That shows that the query had to examine each individual document. 这表明查询必须检查每个单独的文档。 You aren't using an index on the _id field. 您没有在_id字段上使用索引。 Also depending on which fields you are returning, consider creating an index that allows for a covered query. 另外，根据您要返回的字段，请考虑创建一个允许进行覆盖查询的索引。
A covered query is a query that can be satisfied entirely using an index and does not have to examine any documents.
- all the fields in the query are part of an index, and
- all the fields returned in the results are in the same index.
https://docs.mongodb.com/manual/core/query-optimization/ https://docs.mongodb.com/manual/core/query-optimization/
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