我创建了一个包含1,505,000个文档的Mongo集合。
a键上没有索引，而我正在这样做：
db.test.count({a: {$ne: "some_string"}})
返回金额大约需要1200ms。
当我做同样的事情时，但是这次a键上的 WITH 索引，大约需要12000毫秒。
技术细节
索引是使用db.test.createIndex({a: 1})
对于等价而言，当使用find代替count时，它将在0.002 ms之后提供结果。
我在本地主机上使用MongoDB。
这是executionStats：
/* 1 */
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "db.test",
"indexFilterSet" : false,
"parsedQuery" : {
"$nor" : [
{
"a" : {
"$eq" : "some_string"
}
}
]
},
"winningPlan" : {
"stage" : "COUNT",
"inputStage" : {
"stage" : "FETCH",
"inputStage" : {
"stage" : "IXSCAN",
"keyPattern" : {
"a" : 1.0
},
"indexName" : "a_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[MinKey, \"some_string\")",
"(\"some_string\", MaxKey]"
]
}
}
}
},
"rejectedPlans" : []
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 10318,
"totalKeysExamined" : 1505000,
"totalDocsExamined" : 1505000,
"executionStages" : {
"stage" : "COUNT",
"nReturned" : 0,
"executionTimeMillisEstimate" : 9916,
"works" : 1505001,
"advanced" : 0,
"needTime" : 1505000,
"needYield" : 0,
"saveState" : 12102,
"restoreState" : 12102,
"isEOF" : 1,
"invalidates" : 0,
"nCounted" : 1505000,
"nSkipped" : 0,
"inputStage" : {
"stage" : "FETCH",
"nReturned" : 1505000,
"executionTimeMillisEstimate" : 9736,
"works" : 1505001,
"advanced" : 1505000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 12102,
"restoreState" : 12102,
"isEOF" : 1,
"invalidates" : 0,
"docsExamined" : 1505000,
"alreadyHasObj" : 0,
"inputStage" : {
"stage" : "IXSCAN",
"nReturned" : 1505000,
"executionTimeMillisEstimate" : 2479,
"works" : 1505001,
"advanced" : 1505000,
"needTime" : 0,
"needYield" : 0,
"saveState" : 12102,
"restoreState" : 12102,
"isEOF" : 1,
"invalidates" : 0,
"keyPattern" : {
"a" : 1.0
},
"indexName" : "a_1",
"isMultiKey" : false,
"multiKeyPaths" : {
"a" : []
},
"isUnique" : false,
"isSparse" : false,
"isPartial" : false,
"indexVersion" : 1,
"direction" : "forward",
"indexBounds" : {
"a" : [
"[MinKey, \"some_string\")",
"(\"some_string\", MaxKey]"
]
},
"keysExamined" : 1505000,
"seeks" : 1,
"dupsTested" : 0,
"dupsDropped" : 0,
"seenInvalidated" : 0
}
}
}
},
"serverInfo" : {
"host" : "razs-MacBook",
"port" : 27017,
"version" : "3.4.6",
"gitVersion" : ""
},
"ok" : 1.0
}
怎么了？