【发布时间】：2020-06-12 10:40:38
【问题描述】：
更新：我创建了一张票：https://jira.mongodb.org/browse/SERVER-48777
请看下面的例子。为什么第二个查询会执行文档查找？
db.createCollection('inventory')
db.getCollection('inventory').insertMany([
{ type: 'food', color: 'yellow', name: 'banana' },
{ type: 'food', color: 'red', name: 'cherry' },
{ type: 'car', color: 'yellow', name: 'taxi' }
])
db.getCollection('inventory').createIndex({ type: 1, name: 1 }, { name: 'byType' })
db.getCollection('inventory').createIndex({ name: 1 }, { name: 'onlyRed', partialFilterExpression: { color: 'red' } })
// find cherry by type -> totalDocsExamined = 0 (as expected)
db.getCollection('inventory').find({ type: 'food', name: /c/ }, { name: 1, _id: 0 }).explain('executionStats').executionStats.totalDocsExamined
// find cherry by color -> totalDocsExamined = 1 :-(
db.getCollection('inventory').find({ color: 'red', name: /c/ }, { name: 1, _id: 0 }).explain('executionStats').executionStats.totalDocsExamined
上面例子的索引是：
db.getCollection('inventory').getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_"
},
{
"v" : 2,
"key" : {
"type" : 1.0,
"name" : 1.0
},
"name" : "byType"
},
{
"v" : 2,
"key" : {
"name" : 1.0
},
"name" : "onlyRed",
"partialFilterExpression" : {
"color" : "red"
}
}
]
以下所有版本都显示此行为：4.2.1、4.2.7、4.4.0-rc9
我在 Covered Queries 和 Partial Indexes 的文档中找不到任何内容，为什么不应该支持它。
查询1的解释：
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "demo.inventory",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"type" : {
"$eq" : "food"
}
},
{
"name" : {
"$regex" : "c"
}
}
]
},
"winningPlan" : {
"stage" : "PROJECTION_COVERED",
"transformBy" : {
"name" : 1.0,
"_id" : 0.0
},
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"name" : {
"$regex" : "c"
}
},
"keyPattern" : {
"type" : 1.0,
"name" : 1.0
},
"indexName" : "byType",
"isMultiKey" : false,
"multiKeyPaths" : {
"type" : [],
"name" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"type" : [
"[\"food\", \"food\"]"
],
"name" : [
"[\"\", {})",
"[/c/, /c/]"
]
}
}
},
"rejectedPlans" : []
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 0,
"totalKeysExamined" : 2,
"totalDocsExamined" : 0,
"executionStages" : {
"stage" : "PROJECTION_COVERED",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 3,
"advanced" : 1,
"needTime" : 1,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"transformBy" : {
"name" : 1.0,
"_id" : 0.0
},
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"name" : {
"$regex" : "c"
}
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 3,
"advanced" : 1,
"needTime" : 1,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"keyPattern" : {
"type" : 1.0,
"name" : 1.0
},
"indexName" : "byType",
"isMultiKey" : false,
"multiKeyPaths" : {
"type" : [],
"name" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"type" : [
"[\"food\", \"food\"]"
],
"name" : [
"[\"\", {})",
"[/c/, /c/]"
]
},
"keysExamined" : 2,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
},
"serverInfo" : {
"host" : "mongo",
"port" : 27017,
"version" : "4.4.0-rc9",
"gitVersion" : "bea79f76addfe4b754c8696db029c5b3c762041c"
},
"ok" : 1.0,
"$clusterTime" : {
"clusterTime" : Timestamp(1591984674, 1),
"signature" : {
"hash" : { "$binary" : "AAAAAAAAAAAAAAAAAAAAAAAAAAA=", "$type" : "00" },
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1591984674, 1)
}
对query2的解释（使用部分索引）：
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "demo.inventory",
"indexFilterSet" : false,
"parsedQuery" : {
"$and" : [
{
"color" : {
"$eq" : "red"
}
},
{
"name" : {
"$regex" : "c"
}
}
]
},
"winningPlan" : {
"stage" : "PROJECTION_SIMPLE",
"transformBy" : {
"name" : 1.0,
"_id" : 0.0
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"color" : {
"$eq" : "red"
}
},
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"name" : {
"$regex" : "c"
}
},
"keyPattern" : {
"name" : 1.0
},
"indexName" : "onlyRed",
"isMultiKey" : false,
"multiKeyPaths" : {
"name" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : true,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"name" : [
"[\"\", {})",
"[/c/, /c/]"
]
}
}
}
},
"rejectedPlans" : []
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 1,
"executionTimeMillis" : 0,
"totalKeysExamined" : 1,
"totalDocsExamined" : 1,
"executionStages" : {
"stage" : "PROJECTION_SIMPLE",
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"transformBy" : {
"name" : 1.0,
"_id" : 0.0
},
"inputStage" : {
"stage" : "FETCH",
"filter" : {
"color" : {
"$eq" : "red"
}
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"docsExamined" : 1,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"filter" : {
"name" : {
"$regex" : "c"
}
},
"nReturned" : 1,
"executionTimeMillisEstimate" : 0,
"works" : 2,
"advanced" : 1,
"needTime" : 0,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"keyPattern" : {
"name" : 1.0
},
"indexName" : "onlyRed",
"isMultiKey" : false,
"multiKeyPaths" : {
"name" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : true,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"name" : [
"[\"\", {})",
"[/c/, /c/]"
]
},
"keysExamined" : 1,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0
}
}
}
},
"serverInfo" : {
"host" : "mongo",
"port" : 27017,
"version" : "4.4.0-rc9",
"gitVersion" : "bea79f76addfe4b754c8696db029c5b3c762041c"
},
"ok" : 1.0,
"$clusterTime" : {
"clusterTime" : Timestamp(1591984064, 1),
"signature" : {
"hash" : { "$binary" : "AAAAAAAAAAAAAAAAAAAAAAAAAAA=", "$type" : "00" },
"keyId" : NumberLong(0)
}
},
"operationTime" : Timestamp(1591984064, 1)
}
【问题讨论】：
-
我完全不熟悉 Mongo 中的索引，但我可以告诉你，一般来说，任何数据库都可以选择不使用索引，如果这样做不会提高性能的话。至少对于关系 SQL 来说是这样。
-
这个索引创建语句正确吗？
db.getCollection('inventory').createIndex({ name: 1 }, { name: 'onlyRed' }, { partialFilterExpression: { color: 'red' } })。是否值得检查您的索引以确保您正在创建您想要的索引？ -
@willis 在我的示例中确实有一个错字。对此感到抱歉。我纠正了它。我的真实收藏的索引是不同的。我试图在这里找到一个最小的例子。
-
两个查询的完整查询计划是什么？
-
似乎使用了索引，然后服务器执行了一个不需要的过滤阶段。我会将此报告给jira.mongodb.org/browse/SERVER。
标签： mongodb mongodb-query mongodb-indexes