MongoDb Node.js并发查询速度较慢

Question

当我使用 Node.js 运行并发 MongoDb 查询时，第二个查询总是需要 ~2 秒才能返回。使用explain()，executionTimeMillis总是返回 0ms，这绝对是正常的，因为我的测试集合只有 2 个条目。这是我简化的测试用例：

'use strict'
const { MongoClient } = require('mongodb')
const main = async () => {
const client = new MongoClient('mongodb://admin:123456@localhost:27017/', {
useNewUrlParser: true,
useUnifiedTopology: true,
})
await client.connect()
const db = client.db('test')
const numbers = db.collection('numbers')
const promises = []
console.time()
for (let i = 0; i < 3; i++) {
promises.push(numbers.find({ number: i }).explain())
}
for (const promise of promises) {
console.log(await promise)
console.timeLog()
}
console.timeEnd()
await client.close()
}
main()

输出：

{
queryPlanner: {
plannerVersion: 1,
namespace: 'test.numbers',
indexFilterSet: false,
parsedQuery: { number: [Object] },
winningPlan: { stage: 'FETCH', inputStage: [Object] },
rejectedPlans: []
},
executionStats: {
executionSuccess: true,
nReturned: 1,
executionTimeMillis: 0,
totalKeysExamined: 1,
totalDocsExamined: 1,
executionStages: {
stage: 'FETCH',
nReturned: 1,
executionTimeMillisEstimate: 0,
works: 2,
advanced: 1,
needTime: 0,
needYield: 0,
saveState: 0,
restoreState: 0,
isEOF: 1,
invalidates: 0,
docsExamined: 1,
alreadyHasObj: 0,
inputStage: [Object]
},
allPlansExecution: []
},
serverInfo: {
host: 'DESKTOP-C7CAL9N',
port: 27017,
version: '4.0.10',
gitVersion: 'c389e7f69f637f7a1ac3cc9fae843b635f20b766'
},
ok: 1
}
default: 32.252ms
{
queryPlanner: {
plannerVersion: 1,
namespace: 'test.numbers',
indexFilterSet: false,
parsedQuery: { number: [Object] },
winningPlan: { stage: 'FETCH', inputStage: [Object] },
rejectedPlans: []
},
executionStats: {
executionSuccess: true,
nReturned: 1,
executionTimeMillis: 0,
totalKeysExamined: 1,
totalDocsExamined: 1,
executionStages: {
stage: 'FETCH',
nReturned: 1,
executionTimeMillisEstimate: 0,
works: 2,
advanced: 1,
needTime: 0,
needYield: 0,
saveState: 0,
restoreState: 0,
isEOF: 1,
invalidates: 0,
docsExamined: 1,
alreadyHasObj: 0,
inputStage: [Object]
},
allPlansExecution: []
},
serverInfo: {
host: 'DESKTOP-C7CAL9N',
port: 27017,
version: '4.0.10',
gitVersion: 'c389e7f69f637f7a1ac3cc9fae843b635f20b766'
},
ok: 1
}
default: 2042.929ms
{
queryPlanner: {
plannerVersion: 1,
namespace: 'test.numbers',
indexFilterSet: false,
parsedQuery: { number: [Object] },
winningPlan: { stage: 'FETCH', inputStage: [Object] },
rejectedPlans: []
},
executionStats: {
executionSuccess: true,
nReturned: 0,
executionTimeMillis: 0,
totalKeysExamined: 0,
totalDocsExamined: 0,
executionStages: {
stage: 'FETCH',
nReturned: 0,
executionTimeMillisEstimate: 0,
works: 1,
advanced: 0,
needTime: 0,
needYield: 0,
saveState: 0,
restoreState: 0,
isEOF: 1,
invalidates: 0,
docsExamined: 0,
alreadyHasObj: 0,
inputStage: [Object]
},
allPlansExecution: []
},
serverInfo: {
host: 'DESKTOP-C7CAL9N',
port: 27017,
version: '4.0.10',
gitVersion: 'c389e7f69f637f7a1ac3cc9fae843b635f20b766'
},
ok: 1
}
default: 2062.851ms
default: 2063.513ms

如果我按顺序运行查询，则每个查询只需几毫秒即可返回。那为什么是2秒的响应时间呢？

编辑：

在第一个 for 循环中，我promises.push(numbers.find({ number: i }).explain())/运行"并发"查询。在第二个 for 循环中，我等待承诺一个接一个地解决，但这并不意味着一个承诺必须等到前一个承诺解决才能开始它的工作。

为了避免误解，我对我的代码进行了一些更改，将两个 for 循环替换为以下内容：

for (let i = 0; i < 3; i++) {
promises.push(
numbers
.find({ number: i })
.explain()
.then(result => {
// console.log(result)
console.log('query index:', i)
console.timeLog()
})
)
}
await Promise.all(promises)

输出：

query index: 0
default: 22.040ms
query index: 2
default: 2032.921ms
query index: 1
default: 2034.682ms
default: 2035.260ms

编辑 2：

为了进一步澄清，我使用标签来表示计时器。

for (let i = 0; i < 3; i++) {
console.time(`query index: ${ i }`)
promises.push(
numbers
.find({ number: i })
.explain()
.then(result => {
// console.log(result)
console.timeEnd(`query index: ${ i }`)
})
)
}
await Promise.all(promises)

输出：

query index: 0: 12.692ms
query index: 1: 2015.143ms
query index: 2: 2015.310ms

Answer 1

将MongoClient的poolSize设置为1。