我有一个用户的以下模型
{
"TenantId":"abc",
"ContainerId": "xyz",
"Name": "yyy",
"Tags": [
{
"Key":"key1",
"Value":"val1".
}
.....
]
}
TenantId 和 containerId 是 1:n 的关系，因此对于一个 Tenant，可能有多个 ContainerId 项。
我读过，为了获得最大的索引使用率，索引中的第一个字段应该是最具选择性的，以尽量减少 MongoDb 需要扫描的文档。更重要的是，一些人建议最好在该字段上使用“平等”过滤器，而不是范围过滤器。现在，我添加了 2 个索引：
{
"ContanerId":1,
"Tags.Key": 1
}
{
"TenantId":1,
"Tags.Key": 1
}
现在执行 2 个查询，它们返回的结果相等：
find({"TenantId":"abc", "Tags.Key": {$in : ["1","2"]}})
find({"ContainerId":{$in:["abc","xyz",.....]}, "Tags.Key": {$in : ["1","2"]}})
我假设第一个会运行得更快，因为它需要在扫描索引时检查更少的元素。但是，结果似乎几乎相同。有人可以帮我解释一下为什么两个查询的性能相似吗？
第一次查询统计：
{
"executionSuccess":true,
"nReturned":171240,
"executionTimeMillis":602,
"totalKeysExamined":171240,
"totalDocsExamined":171240,
"executionStages":{
"stage":"FETCH",
"nReturned":171240,
"executionTimeMillisEstimate":579,
"works":171241,
"advanced":171240,
"needTime":0,
"needYield":0,
"saveState":1338,
"restoreState":1338,
"isEOF":1,
"invalidates":0,
"docsExamined":171240,
"alreadyHasObj":0,
"inputStage":{
"stage":"IXSCAN",
"nReturned":171240,
"executionTimeMillisEstimate":200,
"works":171241,
"advanced":171240,
"needTime":0,
"needYield":0,
"saveState":1338,
"restoreState":1338,
"isEOF":1,
"invalidates":0,
"keyPattern":{
"PyraTenantId":1,
"IsEmpty":1,
"Tags.KeyValue":1
},
"indexName":"PyraTenantIsEmptyTagsKeyValue",
"isMultiKey":true,
"multiKeyPaths":{
"PyraTenantId":[
],
"IsEmpty":[
],
"Tags.KeyValue":[
"Tags"
]
},
"isUnique":false,
"isSparse":false,
"isPartial":false,
"indexVersion":2,
"direction":"forward",
"indexBounds":{
"PyraTenantId":[
"[\"01fbe4f9-77a5-48a2-8a3d-6b105808f6ac\", \"01fbe4f9-77a5-48a2-8a3d-6b105808f6ac\"]"
],
"IsEmpty":[
"[false, false]"
],
"Tags.KeyValue":[
"[\"01fbe4f9-77a5-48a2-8a3d-6b105808f6ackey6_ddd_01fbe4f9-77a5-48a2-8a3d-6b105808f6acvalue6\", \"01fbe4f9-77a5-48a2-8a3d-6b105808f6ackey6_ddd_01fbe4f9-77a5-48a2-8a3d-6b105808f6acvalue6\"]"
]
},
"keysExamined":171240,
"seeks":1,
"dupsTested":171240,
"dupsDropped":0,
"seenInvalidated":0
}
}
}
第二次查询统计：
{
"executionSuccess" : true,
"nReturned" : 171240,
"executionTimeMillis" : 456,
"totalKeysExamined" : 171412,
"totalDocsExamined" : 171240,
"executionStages" : {
"stage" : "FETCH",
"nReturned" : 171240,
"executionTimeMillisEstimate" : 410,
"works" : 171412,
"advanced" : 171240,
"needTime" : 171,
"needYield" : 0,
"saveState" : 1339,
"restoreState" : 1339,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 171240,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 171240,
"executionTimeMillisEstimate" : 290,
"works" : 171412,
"advanced" : 171240,
"needTime" : 171,
"needYield" : 0,
"saveState" : 1339,
"restoreState" : 1339,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"ContainerId" : 1,
"IsEmpty" : 1,
"Tags.KeyValue" : 1
},
"indexName" : "ContainerIdIsEmptyTagsKeyValue",
"isMultiKey" : true,
"multiKeyPaths" : {
"ContainerId" : [],
"IsEmpty" : [],
"Tags.KeyValue" : [
"Tags"
]
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 2,
"direction" : "forward",
"indexBounds" : {
"ContainerId" : [
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"[\"10fcff07-12aa-4034-b03f-15e6e73cf310\", \"10fcff07-12aa-4034-b03f-15e6e73cf310\"]",
"[\"137997b0-edde-4898-80dd-090f49905d17\", \"137997b0-edde-4898-80dd-090f49905d17\"]",
"[\"1d663037-28b0-4f37-b1ed-ffdaa237b1b9\", \"1d663037-28b0-4f37-b1ed-ffdaa237b1b9\"]",
"[\"25f51039-f3ef-46d8-94b8-a90aa9c8a6d2\", \"25f51039-f3ef-46d8-94b8-a90aa9c8a6d2\"]",
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],
"IsEmpty" : [
"[false, false]"
],
"Tags.KeyValue" : [
"[\"01fbe4f9-77a5-48a2-8a3d-6b105808f6ackey6_ddd_01fbe4f9-77a5-48a2-8a3d-6b105808f6acvalue6\", \"01fbe4f9-77a5-48a2-8a3d-6b105808f6ackey6_ddd_01fbe4f9-77a5-48a2-8a3d-6b105808f6acvalue6\"]"
]
},
"keysExamined" : 171412,
"seeks" : 172,
"dupsTested" : 171240,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}