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我正在尝试对包含公告板数据的集合使用范围查询来获取排序的项目列表。 “线程”文档的数据结构是:
{
"_id" : ObjectId("5a779b47f4fa72412126526a"),
"title" : "necessitatibus tincidunt libris assueverit",
"content" : "Corrumpitvenenatis cubilia adipiscing sollicitudin",
"flagged" : false,
"locked" : false,
"sticky" : false,
"lastPostAt" : ISODate("2018-02-05T06:35:24.656Z"),
"postCount" : 42,
"user" : ObjectId("5a779b46f4fa72412126525a"),
"category" : ObjectId("5a779b31f4fa724121265164"),
"createdAt" : ISODate("2018-02-04T23:46:15.852Z"),
"updatedAt" : ISODate("2018-02-05T06:35:24.656Z")
}
查询是:
db.threads.find({
category: ObjectId('5a779b31f4fa724121265142'),
_id : { $gt: ObjectId('5a779b5cf4fa724121269be8') }
}).sort({ sticky: -1, lastPostAt: -1, _id: 1 }).limit(25)
我设置了以下索引来支持它:
{ category: 1, _id: 1 }
{ category: 1, _id: 1, sticky: 1, lastPostAt: 1 }
{ sticky: 1, lastPostAt: 1, _id: 1 }
尽管如此,它仍然根据执行统计扫描数百个文档/ key :
{
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 772,
"executionTimeMillis" : 17,
"totalKeysExamined" : 772,
"totalDocsExamined" : 772,
"executionStages" : {
"stage" : "SORT",
"nReturned" : 772,
"executionTimeMillisEstimate" : 0,
"works" : 1547,
"advanced" : 772,
"needTime" : 774,
"needYield" : 0,
"saveState" : 33,
"restoreState" : 33,
"isEOF" : 1,
"invalidates" : 0,
"sortPattern" : {
"sticky" : -1,
"lastPostAt" : -1,
"_id" : 1
},
"memUsage" : 1482601,
"memLimit" : 33554432,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"nReturned" : 772,
"executionTimeMillisEstimate" : 0,
"works" : 774,
"advanced" : 772,
"needTime" : 1,
"needYield" : 0,
"saveState" : 33,
"restoreState" : 33,
"isEOF" : 1,
"invalidates" : 0,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 772,
"executionTimeMillisEstimate" : 0,
"works" : 773,
"advanced" : 772,
"needTime" : 0,
"needYield" : 0,
"saveState" : 33,
"restoreState" : 33,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 772,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 772,
"executionTimeMillisEstimate" : 0,
"works" : 773,
"advanced" : 772,
"needTime" : 0,
"needYield" : 0,
"saveState" : 33,
"restoreState" : 33,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"category" : 1,
"_id" : 1,
"sticky" : 1,
"lastPostAt" : 1
},
"indexName" : "category_1__id_1_sticky_1_lastPostAt_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ],
"_id" : [ ],
"sticky" : [ ],
"lastPostAt" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
],
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
],
"sticky" : [
"[MinKey, MaxKey]"
],
"lastPostAt" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 772,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
}
}
当我取出分拣台时,它只正确扫描了 25 个文档。无论我在排序函数中放置哪个字段,检查的键 (772) 都保持不变。
这是排序查询的完整explain()
:
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "database.threads",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"category" : {
"$eq" : ObjectId("5a779b31f4fa724121265142")
}
},
{
"_id" : {
"$gt" : ObjectId("5a779b5cf4fa724121269be8")
}
}
]
},
"winningPlan" : {
"stage" : "SORT",
"sortPattern" : {
"sticky" : -1,
"lastPostAt" : -1,
"_id" : 1
},
"limitAmount" : 25,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"category" : 1,
"_id" : 1,
"sticky" : 1,
"lastPostAt" : 1
},
"indexName" : "category_1__id_1_sticky_1_lastPostAt_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ],
"_id" : [ ],
"sticky" : [ ],
"lastPostAt" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
],
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
],
"sticky" : [
"[MinKey, MaxKey]"
],
"lastPostAt" : [
"[MinKey, MaxKey]"
]
}
}
}
}
},
"rejectedPlans" : [
{
"stage" : "SORT",
"sortPattern" : {
"sticky" : -1,
"lastPostAt" : -1,
"_id" : 1
},
"limitAmount" : 25,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"_id" : {
"$gt" : ObjectId("5a779b5cf4fa724121269be8")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"category" : 1
},
"indexName" : "category_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
]
}
}
}
}
},
{
"stage" : "SORT",
"sortPattern" : {
"sticky" : -1,
"lastPostAt" : -1,
"_id" : 1
},
"limitAmount" : 25,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"category" : 1,
"_id" : 1
},
"indexName" : "category_1__id_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ],
"_id" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
],
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
]
}
}
}
}
},
{
"stage" : "SORT",
"sortPattern" : {
"sticky" : -1,
"lastPostAt" : -1,
"_id" : 1
},
"limitAmount" : 25,
"inputStage" : {
"stage" : "SORT_KEY_GENERATOR",
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"category" : {
"$eq" : ObjectId("5a779b31f4fa724121265142")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
]
}
}
}
}
}
]
},
"serverInfo" : {
"host" : "CRF-MBP.local",
"port" : 27017,
"version" : "3.6.2",
"gitVersion" : "489d177dbd0f0420a8ca04d39fd78d0a2c539420"
},
"ok" : 1
}
下面是非排序查询的完整explain()
:
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "database.threads",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"category" : {
"$eq" : ObjectId("5a779b31f4fa724121265142")
}
},
{
"_id" : {
"$gt" : ObjectId("5a779b5cf4fa724121269be8")
}
}
]
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 25,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"category" : 1,
"_id" : 1,
"sticky" : 1,
"lastPostAt" : 1
},
"indexName" : "category_1__id_1_sticky_1_lastPostAt_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ],
"_id" : [ ],
"sticky" : [ ],
"lastPostAt" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
],
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
],
"sticky" : [
"[MinKey, MaxKey]"
],
"lastPostAt" : [
"[MinKey, MaxKey]"
]
}
}
}
},
"rejectedPlans" : [
{
"stage" : "LIMIT",
"limitAmount" : 25,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"_id" : {
"$gt" : ObjectId("5a779b5cf4fa724121269be8")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"category" : 1
},
"indexName" : "category_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
]
}
}
}
},
{
"stage" : "LIMIT",
"limitAmount" : 25,
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"category" : 1,
"_id" : 1
},
"indexName" : "category_1__id_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"category" : [ ],
"_id" : [ ]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"category" : [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
],
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
]
}
}
}
},
{
"stage" : "LIMIT",
"limitAmount" : 25,
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"category" : {
"$eq" : ObjectId("5a779b31f4fa724121265142")
}
},
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_id" : 1
},
"indexName" : "_id_",
"isMultiKey" : false,
"multiKeyPaths" : {
"_id" : [ ]
},
"isUnique" : true,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"_id" : [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
]
}
}
}
}
]
},
"serverInfo" : {
"host" : "CRF-MBP.local",
"port" : 27017,
"version" : "3.6.2",
"gitVersion" : "489d177dbd0f0420a8ca04d39fd78d0a2c539420"
},
"ok" : 1
}
有谁知道为什么这可能没有完全使用索引?
最佳答案
问题是您的索引都没有实际帮助排序查询。这是大量扫描对象和存在 SORT_KEY_GENERATOR
阶段(内存中排序,限制为 32MB)的原因。
另一方面,非排序查询可以使用 { category: 1, _id: 1 }
或 { category: 1, _id: 1, sticky: 1 , lastPostAt: 1
索引。请注意,使用其中任何一个都是完全有效的,因为一个包含另一个的前缀。参见 Prefixes了解更多详情。
MongoDB find()
查询通常只使用一个索引,所以单个 compound index应该满足您查询的所有参数。这将包括 find()
和 sort()
的参数。
Optimizing MongoDB Compound Indexes 中提供了一篇关于如何创建索引的好文章。 .还是拿文章的重点来说,复合索引排序应该是equality --> sort --> range:
您的查询“形状”是:
db.collection.find({category:..., _id: {$gt:...}})
.sort({sticky:-1, lastPostAt:-1, _id:1})
.limit(25)
我们看到:
category:...
是平等sticky:-1, lastPostAt:-1, _id:1
是排序_id:{$gt:...}
是范围所以你需要的复合索引是:
{category:1, sticky:-1, lastPostAt:-1, _id:1}
使用上述索引的查询的 explain()
输出的获胜计划显示:
"winningPlan": {
"stage": "LIMIT",
"limitAmount": 25,
"inputStage": {
"stage": "FETCH",
"inputStage": {
"stage": "IXSCAN",
"keyPattern": {
"category": 1,
"sticky": -1,
"lastPostAt": -1,
"_id": 1
},
"indexName": "category_1_sticky_-1_lastPostAt_-1__id_1",
"isMultiKey": false,
"multiKeyPaths": {
"category": [ ],
"sticky": [ ],
"lastPostAt": [ ],
"_id": [ ]
},
"isUnique": false,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"category": [
"[ObjectId('5a779b31f4fa724121265142'), ObjectId('5a779b31f4fa724121265142')]"
],
"sticky": [
"[MaxKey, MinKey]"
],
"lastPostAt": [
"[MaxKey, MinKey]"
],
"_id": [
"(ObjectId('5a779b5cf4fa724121269be8'), ObjectId('ffffffffffffffffffffffff')]"
]
}
}
}
}
请注意,获胜计划不包含 SORT_KEY_GENERATOR
阶段。这意味着可以充分利用索引来响应排序后的查询。
关于MongoDB - 排序和限制范围查询时不使用索引,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/48739615/
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