Question

我正在使用mongoose来计算与特定查询匹配的文档数量。我对此查询的索引是：{createdAt: -1, status: -1, oId: -1}

Mongo版本为3.2，该集合中的文档数量约为175万。

如果我这样做：

model.find({
                        createdAt: {'$gte': threeMonths, '$lt': today},
                        status: {'$in': model.STATUS_SET}
                    }).select({_id: 0, status: 1}).count().then((c) => result[alias] = c)

需要2分钟以上。但如果我这样做：

model.find({
                createdAt: {'$gte': threeMonths, '$lt': today},
                status: {'$in': model.STATUS_SET}
            }).select({_id: 0, status: 1}).lean().then((c) => result[alias] = c.length)

然后大约需要2.5秒。

我做错了什么？我能做些什么来帮助加快速度？

编辑：解释日志。

对于计数：

"executionStats" : {
		"executionSuccess" : true,
		"nReturned" : 0,
		"executionTimeMillis" : 82671,
		"totalKeysExamined" : 1749689,
		"totalDocsExamined" : 1643722,
		"executionStages" : {
			"stage" : "COUNT",
			"nReturned" : 0,
			"executionTimeMillisEstimate" : 80960,
			"works" : 1750066,
			"advanced" : 0,
			"needTime" : 1749689,
			"needFetch" : 376,
			"saveState" : 14662,
			"restoreState" : 14662,
			"isEOF" : 1,
			"invalidates" : 0,
			"nCounted" : 1643722,
			"nSkipped" : 0,
			"inputStage" : {
				"stage" : "FETCH",
				"nReturned" : 1643722,
				"executionTimeMillisEstimate" : 80890,
				"works" : 1750065,
				"advanced" : 1643722,
				"needTime" : 105967,
				"needFetch" : 376,
				"saveState" : 14662,
				"restoreState" : 14662,
				"isEOF" : 1,
				"invalidates" : 0,
				"docsExamined" : 1643722,
				"alreadyHasObj" : 0,
				"inputStage" : {
					"stage" : "IXSCAN",
					"nReturned" : 1643722,
					"executionTimeMillisEstimate" : 3800,
					"works" : 1749689,
					"advanced" : 1643722,
					"needTime" : 105967,
					"needFetch" : 0,
					"saveState" : 14662,
					"restoreState" : 14662,
					"isEOF" : 1,
					"invalidates" : 0,
					"keyPattern" : {
						"createdAt" : -1,
						"status" : -1,
						"oId" : -1
					},
					"indexName" : "moderatedContent",
					"isMultiKey" : false,
					"direction" : "forward",
					"indexBounds" : {
						"createdAt" : [
							"(new Date(1467195213000), new Date(1459246413000)]"
						],
						"status" : [
							"[\"UNDECIDED\", \"UNDECIDED\"]",
							"[\"APPROVED\", \"APPROVED\"]"
						],
						"oId" : [
							"[MaxKey, MinKey]"
						]
					},
					"keysExamined" : 1749689,
					"dupsTested" : 0,
					"dupsDropped" : 0,
					"seenInvalidated" : 0,
					"matchTested" : 0
				}
			}
		},
		"allPlansExecution" : [ ]
	}

寻找。

"executionStats" : {
		"executionSuccess" : true,
		"nReturned" : 1643722,
		"executionTimeMillis" : 1216,
		"totalKeysExamined" : 1749689,
		"totalDocsExamined" : 0,
		"executionStages" : {
			"stage" : "PROJECTION",
			"nReturned" : 1643722,
			"executionTimeMillisEstimate" : 1080,
			"works" : 1749690,
			"advanced" : 1643722,
			"needTime" : 105967,
			"needFetch" : 0,
			"saveState" : 13669,
			"restoreState" : 13669,
			"isEOF" : 1,
			"invalidates" : 0,
			"transformBy" : {
				"_id" : 0,
				"status" : 1
			},
			"inputStage" : {
				"stage" : "IXSCAN",
				"nReturned" : 1643722,
				"executionTimeMillisEstimate" : 920,
				"works" : 1749690,
				"advanced" : 1643722,
				"needTime" : 105967,
				"needFetch" : 0,
				"saveState" : 13669,
				"restoreState" : 13669,
				"isEOF" : 1,
				"invalidates" : 0,
				"keyPattern" : {
					"createdAt" : -1,
					"status" : -1,
					"oId" : -1
				},
				"indexName" : "moderatedContent",
				"isMultiKey" : false,
				"direction" : "forward",
				"indexBounds" : {
					"createdAt" : [
						"(new Date(1467195213000), new Date(1459246413000)]"
					],
					"status" : [
						"[\"UNDECIDED\", \"UNDECIDED\"]",
						"[\"APPROVED\", \"APPROVED\"]"
					],
					"oId" : [
						"[MaxKey, MinKey]"
					]
				},
				"keysExamined" : 1749689,
				"dupsTested" : 0,
				"dupsDropped" : 0,
				"seenInvalidated" : 0,
				"matchTested" : 0
			}
		}
	}

首先在光标中使用.explain（）发布：

"executionStats" : {
		"executionSuccess" : true,
		"nReturned" : 0,
		"executionTimeMillis" : 89191,
		"totalKeysExamined" : 1749689,
		"totalDocsExamined" : 1643722,
		"executionStages" : {
			"stage" : "COUNT",
			"nReturned" : 0,
			"executionTimeMillisEstimate" : 83400,
			"works" : 1751709,
			"advanced" : 0,
			"needTime" : 1749689,
			"needFetch" : 2019,
			"saveState" : 15648,
			"restoreState" : 15648,
			"isEOF" : 1,
			"invalidates" : 0,
			"nCounted" : 1643722,
			"nSkipped" : 0,
			"inputStage" : {
				"stage" : "FETCH",
				"nReturned" : 1643722,
				"executionTimeMillisEstimate" : 83260,
				"works" : 1751708,
				"advanced" : 1643722,
				"needTime" : 105967,
				"needFetch" : 2019,
				"saveState" : 15648,
				"restoreState" : 15648,
				"isEOF" : 1,
				"invalidates" : 0,
				"docsExamined" : 1643722,
				"alreadyHasObj" : 0,
				"inputStage" : {
					"stage" : "IXSCAN",
					"nReturned" : 1643722,
					"executionTimeMillisEstimate" : 8290,
					"works" : 1749689,
					"advanced" : 1643722,
					"needTime" : 105967,
					"needFetch" : 0,
					"saveState" : 15648,
					"restoreState" : 15648,
					"isEOF" : 1,
					"invalidates" : 0,
					"keyPattern" : {
						"createdAt" : -1,
						"status" : -1,
						"oId" : -1
					},
					"indexName" : "moderatedContent",
					"isMultiKey" : false,
					"direction" : "forward",
					"indexBounds" : {
						"createdAt" : [
							"(new Date(1467195213000), new Date(1459246413000)]"
						],
						"status" : [
							"[\"UNDECIDED\", \"UNDECIDED\"]",
							"[\"APPROVED\", \"APPROVED\"]"
						],
						"oId" : [
							"[MaxKey, MinKey]"
						]
					},
					"keysExamined" : 1749689,
					"dupsTested" : 0,
					"dupsDropped" : 0,
					"seenInvalidated" : 0,
					"matchTested" : 0
				}
			}
		}
	}

Answer 1

答案的关键是

//count
"totalDocsExamined" : 1643722,

VS

//find
"totalDocsExamined" : 0,

查询查询完全在索引上运行，并且在count查询实际读取db中的每个文档时不会读取任何单个文档。

原因是，您的查找查询使用lean()选项。根据猫鼬doc：

启用精益选项的查询返回的文档是纯JavaScript对象，而不是MongooseDocuments。他们没有使用保存方法，getter / setter或其他Mongoose魔法。

最重要的是，在您的精简查找查询中，您只有select() id和status，这似乎是预测（"transformBy" ...）所以整个查询变为 covered query ，并且不得读取任何文档来提供请求。

Answer 2

查找 - 查找集合中的文档并将光标返回到所选文档。 Cursor是指向查询结果集的指针。客户端可以遍历游标以检索结果。

Count - count（）等同于db.collection.find(query).count()

Count实际上是一个游标方法，shell只是提供了一个快捷方式。在Shell中，我们有计数快捷方式，如db.collection.count(query)

所示

原因：在查找操作中，我们没有迭代结果集，而在count操作中，我们正在对结果集执行操作。那会导致时间延迟。

https://docs.mongodb.com/manual/reference/method/db.collection.find/ https://docs.mongodb.com/manual/reference/method/db.collection.count/

mongodb - count（）比find（）慢得多吗？

2 个答案: