为什么多个 sql 查询运行是按顺序而不是同时进行的？

Question

我正在运行ning mysql “select” 同时在单独的 go 例程中查询。然而，mysql 服务似乎收集这些查询，运行按顺序（不是同时）然后 returns 同时收集所有结果集，并且只有在所有查询都被运行.

我的问题是：
1.为什么这些查询运行是顺序的而不是并发的？
2. 为什么 mysql 等到所有查询都已运行才同时返回每个查询的结果集（即使每个结果集属于不同的 go routine 并且据说也使用单独的连接)?

另一件事：当我设置“SetMaxOpenConns(2)”时，它会同时 returns 两个结果集。如果设置为 3，则同时返回 3 个结果集。但是它们仍然总是运行顺序。

有人知道这是怎么回事吗？

package main

import (
    "database/sql"
    "fmt"
    "sync"
    "time"

    _ "github.com/go-sql-driver/mysql"
)

var database *sql.DB

func init() {
    var err error
    databaseURI := "root:toor@tcp(192.168.200.10:3306)/ahc"
    database, err = sql.Open("mysql", databaseURI)
    if err != nil {
        fmt.Println(err)
    } else {
        fmt.Println("DB Connection Established")
        //database.SetMaxIdleConns(0)
        //database.SetMaxOpenConns(2)
    }
}

func test(device string, wg *sync.WaitGroup) {
    fmt.Println("Thread: " + device + " started")
    start := time.Now()

    var count string

    //using go-sql-driver
    sqlStatement, err := database.Prepare("select count(cpeName) from report where errMessage <> \"ok\" and cpeName = ? and jobID = ?")
    if err != nil {
        fmt.Println(err)
    }
    defer sqlStatement.Close()
    err = sqlStatement.QueryRow(device, "11534").Scan(&count)
    sqlStatement.Close()


    duration := time.Since(start)
    fmt.Println("Thread: " + device + " Duration: " + duration.String() + "\n")
    wg.Done()
}

func main() {
    var wg sync.WaitGroup
    var deviceList = []string{"xx-swrk-ca-gen-s-002", "xx-leus-ca-ust-ap-068", "xx-sgvn-ca-lug-ap-004", "xx-swrk-ca-vez-s-005", "xx-swrk-ca-vez-ap-006",    "xx-leus-ca-ust-ap-065", "xx-leus-ca-ust-ap-073", "xx-leus-ca-ust-ap-076", "xx-leus-ca-ust-ap-077", "xx-swrk-ca-gen-s-001"}
    total := time.Now()
    for _, device := range deviceList {
        wg.Add(1)
        go test(device, &wg)
    }
    wg.Wait()
    duration := time.Since(total)
    fmt.Println("\n\nTotal: Duration: " + duration.String() + "\n")
}

这是输出

DB Connection Established
Thread: xx-leus-ca-ust-ap-068 started
Thread: xx-sgvn-ca-lug-ap-004 started
Thread: xx-leus-ca-ust-ap-065 started
Thread: xx-leus-ca-ust-ap-073 started
Thread: xx-swrk-ca-vez-ap-006 started
Thread: xx-swrk-ca-vez-s-005 started
Thread: xx-leus-ca-ust-ap-076 started
Thread: xx-leus-ca-ust-ap-077 started
Thread: xx-swrk-ca-gen-s-002 started
Thread: xx-swrk-ca-gen-s-001 started
Thread: xx-leus-ca-ust-ap-076 Duration: 7.291656143s

Thread: xx-swrk-ca-gen-s-002 Duration: 7.304134404s

Thread: xx-leus-ca-ust-ap-065 Duration: 7.307958641s

Thread: xx-swrk-ca-vez-s-005 Duration: 7.313591747s

Thread: xx-leus-ca-ust-ap-077 Duration: 7.313992638s

Thread: xx-swrk-ca-vez-ap-006 Duration: 7.314905664s

Thread: xx-swrk-ca-gen-s-001 Duration: 7.320466323s

Thread: xx-leus-ca-ust-ap-073 Duration: 7.322158337s

Thread: xx-leus-ca-ust-ap-068 Duration: 7.324745097s

Thread: xx-sgvn-ca-lug-ap-004 Duration: 7.326001783s



Total: Duration: 7.326096238s

当使用 database.SetMaxOpenConns(1) 时，这是输出：

DB Connection Established
Thread: xx-leus-ca-ust-ap-068 started
Thread: xx-swrk-ca-gen-s-001 started
Thread: xx-swrk-ca-vez-ap-006 started
Thread: xx-leus-ca-ust-ap-065 started
Thread: xx-leus-ca-ust-ap-073 started
Thread: xx-swrk-ca-gen-s-002 started
Thread: xx-leus-ca-ust-ap-077 started
Thread: xx-sgvn-ca-lug-ap-004 started
Thread: xx-leus-ca-ust-ap-076 started
Thread: xx-swrk-ca-vez-s-005 started
Thread: xx-leus-ca-ust-ap-068 Duration: 1.131790286s

Thread: xx-leus-ca-ust-ap-077 Duration: 2.128919333s

Thread: xx-swrk-ca-gen-s-001 Duration: 3.073559464s

Thread: xx-leus-ca-ust-ap-073 Duration: 4.002964333s

Thread: xx-swrk-ca-vez-s-005 Duration: 4.932256684s

Thread: xx-sgvn-ca-lug-ap-004 Duration: 5.853361245s

Thread: xx-swrk-ca-gen-s-002 Duration: 6.785042625s

Thread: xx-leus-ca-ust-ap-065 Duration: 7.705957815s

Thread: xx-swrk-ca-vez-ap-006 Duration: 8.633000734s

Thread: xx-leus-ca-ust-ap-076 Duration: 9.550948572s



Total: Duration: 9.551103129s

Answer 1

查看它们是运行并行还是串行的简单技术：让每个连接都做

SELECT SLEEP(1);

如果并行，设置不会超过 1 秒。如果是连续的，那么N秒。

如果它们是运行顺序的，但是直到全部完成才输出，那就是GO问题。

如果您的 7.3s 确实是并行的而 9.5 是串行的，那么这就指出了为什么运行并行的东西不值得——它们会相互踩踏并且不会更快地完成。冲突可能是 CPU 或 I/O 或互斥体或网络或其他东西；这取决于查询。（我的 sleep 测试非常 non-invasive，但需要的时间是可预测的。）

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