我很难说服Mongo运行distinct查询，看起来它应该被索引覆盖，而不会在集合中获取大量文档。
我的文件有一般形式：
{
_tenantId: 'someString',
_productCategory: 'some string from a smallish set'
...
}
我有一个索引（_tenantId，_productCategory）。
我想知道给定租户的不同产品类别是什么，所以查询是：
db.products.distinct( '_productCategory', { _tenantId: '463171c3-d15f-4699-893d-3046327f8e1f'})
这种运行速度相当缓慢（对于本地数据库，即Mongo 3.2.9，收集大约50万个产品需要几秒钟）。对于我们预先生产的基于SaaS的Mongo（可能比我的本地实例更多的内存限制，它可以自由运行我的机器），相同的数据需要几十秒的时间。
解释查询产生：
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "engage-prod.products",
"indexFilterSet" : false,
"parsedQuery" : {
"_tenantId" : {
"$eq" : "463171c3-d15f-4699-893d-3046327f8e1f"
}
},
"winningPlan" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"_tenantId" : 1,
"_productCategory" : 1
},
"indexName" : "_tenantId_1__productCategory_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"_tenantId" : [
"[\"463171c3-d15f-4699-893d-3046327f8e1f\", \"463171c3-d15f-4699-893d-3046327f8e1f\"]"
],
"_productCategory" : [
"[MinKey, MaxKey]"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 406871,
"executionTimeMillis" : 358,
"totalKeysExamined" : 406871,
"totalDocsExamined" : 406871,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 406871,
"executionTimeMillisEstimate" : 80,
"works" : 406872,
"advanced" : 406871,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3178,
"restoreState" : 3178,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 406871,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 406871,
"executionTimeMillisEstimate" : 40,
"works" : 406872,
"advanced" : 406871,
"needTime" : 0,
"needYield" : 0,
"saveState" : 3178,
"restoreState" : 3178,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"_tenantId" : 1,
"_productCategory" : 1
},
"indexName" : "_tenantId_1__productCategory_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"_tenantId" : [
"[\"463171c3-d15f-4699-893d-3046327f8e1f\", \"463171c3-d15f-4699-893d-3046327f8e1f\"]"
],
"_productCategory" : [
"[MinKey, MaxKey]"
]
},
"keysExamined" : 406871,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
},
"serverInfo" : {
"host" : "Stevens-MacBook-Pro.local",
"port" : 27017,
"version" : "3.2.9",
"gitVersion" : "22ec9e93b40c85fc7cae7d56e7d6a02fd811088c"
},
"ok" : 1
}
请注意，即使它运行IXSCAN，它仍然会返回超过400K的文档（nReturned）。
如果我创建一个包含词汇连接（带有_tenantAndProductCategory分隔符）的索引的复合字段:和索引，使其成为单个字段索引，那么查询：
db.products.explain('executionStats').distinct( '_productTenantAndCategory', { _productTenantAndCategory: {$gte: '463171c3-d15f-4699-893d-3046327f8e1f',$lt: '463171c3-d15f-4699-893d-3046327f8e1g'}})
完全在索引中工作并产生：
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "engage-prod.products",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"_productTenantAndCategory" : {
"$lt" : "463171c3-d15f-4699-893d-3046327f8e1g"
}
},
{
"_productTenantAndCategory" : {
"$gte" : "463171c3-d15f-4699-893d-3046327f8e1f"
}
}
]
},
"winningPlan" : {
"stage" : "PROJECTION",
"transformBy" : {
"_id" : 0,
"_productTenantAndCategory" : 1
},
"inputStage" : {
"stage" : "DISTINCT_SCAN",
"keyPattern" : {
"_productTenantAndCategory" : 1
},
"indexName" : "_productTenantAndCategory_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"_productTenantAndCategory" : [
"[\"463171c3-d15f-4699-893d-3046327f8e1f\", \"463171c3-d15f-4699-893d-3046327f8e1g\")"
]
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 62,
"executionTimeMillis" : 0,
"totalKeysExamined" : 63,
"totalDocsExamined" : 0,
"executionStages" : {
"stage" : "PROJECTION",
"nReturned" : 62,
"executionTimeMillisEstimate" : 0,
"works" : 63,
"advanced" : 62,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"transformBy" : {
"_id" : 0,
"_productTenantAndCategory" : 1
},
"inputStage" : {
"stage" : "DISTINCT_SCAN",
"nReturned" : 62,
"executionTimeMillisEstimate" : 0,
"works" : 63,
"advanced" : 62,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"_productTenantAndCategory" : 1
},
"indexName" : "_productTenantAndCategory_1",
"isMultiKey" : false,
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"_productTenantAndCategory" : [
"[\"463171c3-d15f-4699-893d-3046327f8e1f\", \"463171c3-d15f-4699-893d-3046327f8e1g\")"
]
},
"keysExamined" : 63
}
}
},
"serverInfo" : {
"host" : "Stevens-MacBook-Pro.local",
"port" : 27017,
"version" : "3.2.9",
"gitVersion" : "22ec9e93b40c85fc7cae7d56e7d6a02fd811088c"
},
"ok" : 1
}
我需要为我需要的所有聚合查询构建具有手动复合键的单个字段索引，这不是一个非常理想的路径。由于所有信息都出现在我开始使用的复合索引中，为什么Mongo执行原始distinct查询并覆盖该索引？我有什么办法可以通过查询优化来克服这个问题吗？
注意这实际上是一个稍微复杂的子问题，涉及一个聚合管道来实际计算每个类别的出现次数，但我现在将我的问题限制为更简单distinct查询，因为它似乎捕获了使用应覆盖事物的索引（我在聚合管道案例中也看到）的失败的本质，同时是一个更简单的整体查询。