unity线程不随主线程关闭

关于线程的一点困惑，我写了个脚本批量往数据库里填数据。直接一股脑填卡电脑，所以开了个线程让它一点点往里填，但是我发现我unity停止线程仍在执行（debug一直在输出且数据库不断有数据加入）。我没有写线程关闭的逻辑，因为我理解的unity里面的线程都属于子线程，会随着unity主线程的关闭而关闭，所以出现这一幕挺困惑的，也找不到太好的说明，希望大家帮忙解惑。

using System.Collections;
using System.Collections.Generic;
using MySql.Data.MySqlClient;
using UnityEngine;
using System.IO;
using System.Threading;
public class MysqlOp : MonoBehaviour
{
    const string firt = "-";
    const string second = "--";
    const string third = "---";
    int id = 0;
    int parentID = -1;
    int parentID2=-1;
    MySqlConnection conn;

    // Start is called before the first frame update
    void Start()
    {
        ConnectMysql();
        Thread t=new Thread(ReadFile);
        t.Start();
    }

    // Update is called once per frame
    void Update()
    {

    }
    void ReadFile()
    {
        string path = Application.streamingAssetsPath + "/1.txt";
        string[] arr = File.ReadAllLines(path);
        for (int i = 0; i < arr.Length; i++)
        {
            id++;
            string[] temp = arr[i].Split('>');
            if (temp[1].StartsWith(firt) && !temp[1].StartsWith(second) && !temp[1].StartsWith(third))
            {
                parentID=id;
                parentID2=0;
                Debug.Log(temp[1]);
                Insert(id,"0",temp[1].Replace(firt,""),"1");
            }
             if (temp[1].StartsWith(second) && !temp[1].StartsWith(third))
            {
                parentID2=id;
                Debug.Log("二级结构："+temp[1]);

                Insert(id,parentID.ToString(),temp[1].Replace(second,""),"2");
            }
             if (temp[1].StartsWith(third))
            {
                
                Insert(id,parentID2.ToString(),temp[1].Replace(third,""),"3");
            }
            Thread.Sleep(500);
        }
    }

    void ConnectMysql()
    {
        string sqlSer = "server = localhost;port = 3306;database = db_whlearn;user = root;password = root";
        //建立连接
        conn = new MySqlConnection(sqlSer);
        try
        {
            conn.Open();
            Debug.Log("------链接成功------");
            //sql语句
        }
        catch (System.Exception e)
        {
            Debug.Log(e.Message);
        }
       
    }
    void Insert(int id, string pid, string name,string leader)
    {
        
        string sqlQuary = "insert into ums_dept(id,parent_id,dept_name,leader) values (@id, @parent_id,@dept_name,@leader)";

        MySqlCommand comd = new MySqlCommand(sqlQuary, conn);
        comd.Parameters.AddWithValue("@id", id);
        comd.Parameters.AddWithValue("@parent_id", pid);
        comd.Parameters.AddWithValue("@dept_name", name);
        comd.Parameters.AddWithValue("@leader", leader);

        comd.ExecuteNonQuery();
        
    }
}

子线程默认是不随主线程关闭而自动关闭的
你可以在主线程退出的时候执行杀死进程