【发布时间】：2021-09-08 12:38:53
【问题描述】：
我遇到了一个奇怪的问题。查询的解释显示在所有阶段都使用了具有快速执行时间的索引，但 LIMIT 的最后阶段打破了这一切。我的执行时间是 60 多秒！
数据库用于营销工具，我们收集有关活动活动的数据。我们在数据库中每小时有 100k+ 条记录，我想以 5k 的批次选择所有 100k 条记录（我也尝试将其减少到 1k 条），以便进行统计汇总。
只是为了测试，我会添加一个查询解释的例子。
- 如果我将
created_at日期范围减少到 10 分钟，并将limit设置为 1000，它会运行得很快。 - 如果我将
created_at日期范围设置为 30 分钟，并将limit设置为 1000，它会再次存货。 - 我有 8 个 CPU 和 64 个内存
- 存储容量为 6000 IOPS
- 该表总共包含 9 亿条记录
我有以下索引：
- created_at: 1
- _id: 1
- {created_at: 1, _id: 1} - 未在查询中使用
慢查询 30 分钟
> db.logs.explain('allPlansExecution').aggregate([{"$match":{"created_at":{"$gte":ISODate('2021-06-02T20:00:00.000+00:00'),"$lte":ISODate('2021-06-02T20:30:00.000+00:00')}}},{"$sort":{"_id":1}},{"$limit":1000}], { allowDiskUse: true });
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "webpush.campaign_action_logs",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"created_at" : {
"$lte" : ISODate("2021-06-02T20:30:00Z")
}
},
{
"created_at" : {
"$gte" : ISODate("2021-06-02T20:00:00Z")
}
}
]
},
"optimizedPipeline" : true,
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"_id" : 1
},
"memLimit" : 104857600,
"limitAmount" : 100,
"type" : "simple",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"created_at" : 1
},
"indexName" : "created_at_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"created_at" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"created_at" : [
"[new Date(1622664000000), new Date(1622665800000)]"
]
}
}
}
},
"rejectedPlans" : [
{
"stage" : "LIMIT",
"limitAmount" : 100,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"created_at" : {
"$lte" : ISODate("2021-06-02T20:30:00Z")
}
},
{
"created_at" : {
"$gte" : ISODate("2021-06-02T20:00:00Z")
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[MinKey, MaxKey]"
]
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 100,
"executionTimeMillis" : 183411,
"totalKeysExamined" : 257959,
"totalDocsExamined" : 257959,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 100,
"executionTimeMillisEstimate" : 625,
"works" : 258061,
"advanced" : 100,
"needTime" : 257960,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 1,
"sortPattern" : {
"_id" : 1
},
"memLimit" : 104857600,
"limitAmount" : 100,
"type" : "simple",
"totalDataSizeSorted" : 187871472,
"usedDisk" : false,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 257959,
"executionTimeMillisEstimate" : 533,
"works" : 257960,
"advanced" : 257959,
"needTime" : 0,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 1,
"docsExamined" : 257959,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 257959,
"executionTimeMillisEstimate" : 265,
"works" : 257960,
"advanced" : 257959,
"needTime" : 0,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 1,
"keyPattern" : {
"created_at" : 1
},
"indexName" : "created_at_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"created_at" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"created_at" : [
"[new Date(1622664000000), new Date(1622665800000)]"
]
},
"keysExamined" : 257959,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"allPlansExecution" : [
{
"nReturned" : 100,
"executionTimeMillisEstimate" : 625,
"totalKeysExamined" : 257959,
"totalDocsExamined" : 257959,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 100,
"executionTimeMillisEstimate" : 625,
"works" : 258060,
"advanced" : 100,
"needTime" : 257960,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 0,
"sortPattern" : {
"_id" : 1
},
"memLimit" : 104857600,
"limitAmount" : 100,
"type" : "simple",
"totalDataSizeSorted" : 187871472,
"usedDisk" : false,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 257959,
"executionTimeMillisEstimate" : 533,
"works" : 257960,
"advanced" : 257959,
"needTime" : 0,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 1,
"docsExamined" : 257959,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 257959,
"executionTimeMillisEstimate" : 265,
"works" : 257960,
"advanced" : 257959,
"needTime" : 0,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 1,
"keyPattern" : {
"created_at" : 1
},
"indexName" : "created_at_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"created_at" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"created_at" : [
"[new Date(1622664000000), new Date(1622665800000)]"
]
},
"keysExamined" : 257959,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
},
{
"nReturned" : 70,
"executionTimeMillisEstimate" : 178092,
"totalKeysExamined" : 258060,
"totalDocsExamined" : 258060,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 70,
"executionTimeMillisEstimate" : 178092,
"works" : 258060,
"advanced" : 70,
"needTime" : 257990,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 0,
"limitAmount" : 100,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"created_at" : {
"$lte" : ISODate("2021-06-02T20:30:00Z")
}
},
{
"created_at" : {
"$gte" : ISODate("2021-06-02T20:00:00Z")
}
}
]
},
"nReturned" : 70,
"executionTimeMillisEstimate" : 178057,
"works" : 258060,
"advanced" : 70,
"needTime" : 257990,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 0,
"docsExamined" : 258060,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 258060,
"executionTimeMillisEstimate" : 645,
"works" : 258060,
"advanced" : 258060,
"needTime" : 0,
"needYield" : 0,
"saveState" : 9571,
"restoreState" : 9571,
"isEOF" : 0,
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 258060,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
}
]
},
"serverInfo" : {
"host" : "ip-10-0-3-171",
"port" : 27017,
"version" : "4.4.6",
"gitVersion" : "72e66213c2c3eab37d9358d5e78ad7f5c1d0d0d7"
},
"ok" : 1
}
10 分钟的快速查询：
> db.logs.explain('allPlansExecution').aggregate([{"$match":{"created_at":{"$gte":ISODate('2021-06-02T20:00:00.000+00:00'),"$lte":ISODate('2021-06-02T20:10:00.000+00:00')}}},{"$sort":{"_id":1}},{"$limit":1000}], { allowDiskUse: true });
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "webpush.campaign_action_logs",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"created_at" : {
"$lte" : ISODate("2021-06-02T20:10:00Z")
}
},
{
"created_at" : {
"$gte" : ISODate("2021-06-02T20:00:00Z")
}
}
]
},
"optimizedPipeline" : true,
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"_id" : 1
},
"memLimit" : 104857600,
"limitAmount" : 1000,
"type" : "simple",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"created_at" : 1
},
"indexName" : "created_at_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"created_at" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"created_at" : [
"[new Date(1622664000000), new Date(1622664600000)]"
]
}
}
}
},
"rejectedPlans" : [
{
"stage" : "LIMIT",
"limitAmount" : 1000,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"created_at" : {
"$lte" : ISODate("2021-06-02T20:10:00Z")
}
},
{
"created_at" : {
"$gte" : ISODate("2021-06-02T20:00:00Z")
}
}
]
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[MinKey, MaxKey]"
]
}
}
}
}
]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1000,
"executionTimeMillis" : 1502,
"totalKeysExamined" : 58027,
"totalDocsExamined" : 58027,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 1000,
"executionTimeMillisEstimate" : 122,
"works" : 59029,
"advanced" : 1000,
"needTime" : 58028,
"needYield" : 0,
"saveState" : 122,
"restoreState" : 122,
"isEOF" : 1,
"sortPattern" : {
"_id" : 1
},
"memLimit" : 104857600,
"limitAmount" : 1000,
"type" : "simple",
"totalDataSizeSorted" : 42213931,
"usedDisk" : false,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 58027,
"executionTimeMillisEstimate" : 96,
"works" : 58028,
"advanced" : 58027,
"needTime" : 0,
"needYield" : 0,
"saveState" : 122,
"restoreState" : 122,
"isEOF" : 1,
"docsExamined" : 58027,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 58027,
"executionTimeMillisEstimate" : 40,
"works" : 58028,
"advanced" : 58027,
"needTime" : 0,
"needYield" : 0,
"saveState" : 122,
"restoreState" : 122,
"isEOF" : 1,
"keyPattern" : {
"created_at" : 1
},
"indexName" : "created_at_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"created_at" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"created_at" : [
"[new Date(1622664000000), new Date(1622664600000)]"
]
},
"keysExamined" : 58027,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"allPlansExecution" : [
{
"nReturned" : 101,
"executionTimeMillisEstimate" : 122,
"totalKeysExamined" : 58027,
"totalDocsExamined" : 58027,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 101,
"executionTimeMillisEstimate" : 122,
"works" : 58129,
"advanced" : 101,
"needTime" : 58028,
"needYield" : 0,
"saveState" : 121,
"restoreState" : 121,
"isEOF" : 0,
"sortPattern" : {
"_id" : 1
},
"memLimit" : 104857600,
"limitAmount" : 1000,
"type" : "simple",
"totalDataSizeSorted" : 42213931,
"usedDisk" : false,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 58027,
"executionTimeMillisEstimate" : 96,
"works" : 58028,
"advanced" : 58027,
"needTime" : 0,
"needYield" : 0,
"saveState" : 121,
"restoreState" : 121,
"isEOF" : 1,
"docsExamined" : 58027,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 58027,
"executionTimeMillisEstimate" : 40,
"works" : 58028,
"advanced" : 58027,
"needTime" : 0,
"needYield" : 0,
"saveState" : 121,
"restoreState" : 121,
"isEOF" : 1,
"keyPattern" : {
"created_at" : 1
},
"indexName" : "created_at_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"created_at" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"created_at" : [
"[new Date(1622664000000), new Date(1622664600000)]"
]
},
"keysExamined" : 58027,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
},
{
"nReturned" : 3,
"executionTimeMillisEstimate" : 935,
"totalKeysExamined" : 58129,
"totalDocsExamined" : 58129,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 3,
"executionTimeMillisEstimate" : 935,
"works" : 58129,
"advanced" : 3,
"needTime" : 58126,
"needYield" : 0,
"saveState" : 122,
"restoreState" : 122,
"isEOF" : 0,
"limitAmount" : 1000,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"$and" : [
{
"created_at" : {
"$lte" : ISODate("2021-06-02T20:10:00Z")
}
},
{
"created_at" : {
"$gte" : ISODate("2021-06-02T20:00:00Z")
}
}
]
},
"nReturned" : 3,
"executionTimeMillisEstimate" : 935,
"works" : 58129,
"advanced" : 3,
"needTime" : 58126,
"needYield" : 0,
"saveState" : 122,
"restoreState" : 122,
"isEOF" : 0,
"docsExamined" : 58129,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 58129,
"executionTimeMillisEstimate" : 17,
"works" : 58129,
"advanced" : 58129,
"needTime" : 0,
"needYield" : 0,
"saveState" : 122,
"restoreState" : 122,
"isEOF" : 0,
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 58129,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
}
]
},
"serverInfo" : {
"host" : "ip-10-0-3-171",
"port" : 27017,
"version" : "4.4.6",
"gitVersion" : "72e66213c2c3eab37d9358d5e78ad7f5c1d0d0d7"
},
"ok" : 1
}
问题：
-
如果问题是扫描的文档数量并且我们使用了索引，我该如何解决？我不认为循环超过 5000k 会导致 300k 文档中的排序顺序出现问题，但也许我错了。
-
为什么 LIMIT 阶段（带有另一个 FETCH 阶段）会导致问题？
提前非常感谢各位专家！
【问题讨论】：
-
如果没有 sort 阶段，您的查询如何执行（我会尝试这个来理解问题）。
-
@prasad_ 很好的问题，没有我们所说的几毫秒查询的排序。超级快。问题是我必须对结果进行排序，因为我需要在每 5000 个项目之间跳转，并且当您需要跳过 100k 个项目时使用“跳过”和“限制”不是一个好习惯。 sammaye.wordpress.com/2012/05/25/…
-
您的查询的缺点是在 $match 阶段有一个范围过滤器。这不允许 $sort 使用索引。这是进行内存排序的排序阶段 - 它无法使用索引。在$match 阶段 之后返回了多少个文档？ （注意：我没有阅读链接的文章）。
-
嘿，这篇文章只是为了解释为什么我避免跳过/限制。 @prasad_the docs after match 你可以在我在消息中添加的第一个“解释”中看到，在比赛的第一阶段我们有“docsExamined”：257959，我想返回 5000 个项目的限制，然后按 ID 跳过下一个 5000。
-
您可能会受益于在 $match 阶段（连同当前过滤器）添加另一个具有相等条件的过滤器 - 这样我们可以让排序阶段使用索引.
标签： mongodb performance indexing