MySQL returning “Too many connections” error

Question

I'm doing something like this:

import(
    "database/sql"
    "github.com/go-sql-driver/mysql"
)

var db *sql.DB

func main() {
    var err error
    db, err = sql.Open(...)

    if err != nil {
        panic(err)
    }

    for j := 0; j < 8000; j++ {
        _, err := db.Query("QUERY...")
        
        if err != nil {
            logger.Println("Error " + err.Error())
            return
        }
    }
 }

It works for the first 150 queries (for that I'm using another function to make) but after that, I get the error:

mysqli_real_connect(): (HY000/1040): Too many connections

So clearly I'm doing something wrong but I can't find what is it. I don't know what to open and close a new connection for each query.

Error in the log file : 
"reg: 2020/06/28 03:35:34 Errores  Error 1040: Too many connections"
(it is printed only once)

Error in mysql php my admin: 
 "mysqli_real_connect(): (HY000/1040): Too many connections"
 "La conexión para controluser, como está definida en su configuración, fracasó."
(translated: "the connection for controluser, as it is defined in ti's configuration , failed.")
 "mysqli_real_connect(): (08004/1040): Too many connections"

Answer 1

Every time you call Query() , you're creating a new database handle. Each active handle needs a unique database connection. Since you're not calling Close , that handle, and thus the connection, remains open until the program exits.

Solve your problem by calling rows.Close() after you're done with each query:

    for j := 0; j < 8000; j++ {
        rows, err := db.Query("QUERY...")
        if err != nil {
            logger.Println("Error " + err.Error())
            return
        }
        // Your main logic here
        rows.Close()
    }

This Close() call is often called in a defer statement, but this precludes the use of a for loop (since a defer only executes when then function returns), so you may want to move your main logic to a new function:

    for j := 0; j < 8000; j++ {
        doStuff()
    }

// later

func doStuff() {
    rows, err := db.Query("QUERY...")
    if err != nil {
        logger.Println("Error " + err.Error())
        return
    }
    defer rows.Close()
    // Your main logic here
}