从数据库查询20w数据太慢，怎么用多线程并行查询汇总list集合

    @Autowired
    private RedisActiveTestsMapper redisActiveTestsMapper;

public List<List<RedisActiveTests>> selectMult(RedisActiveTests redisActiveTests) {
        System.out.println("----程序开始运行----");
        Date date1 = new Date();
        int taskSize = 10;
        // 创建一个线程池
        ExecutorService pool = Executors.newFixedThreadPool(taskSize);
        // 创建多个有返回值的任务
        List<Future> list = new ArrayList<Future>();
        for (int i = 0; i < taskSize; i++) {
            Callable c = new ThredQuery(redisActiveTestsMapper);
            // 执行任务并获取Future对象
            Future f = pool.submit(c);
            // System.out.println(">>>" + f.get().toString());
            list.add(f);
        }
        // 关闭线程池
        pool.shutdown();

        // 获取所有并发任务的运行结果
        for (Future f : list) {
            // 从Future对象上获取任务的返回值，并输出到控制台
            try {
                String s = f.get().toString();
                System.out.println(">>>" +s );
            }catch (Exception e){

            }

        }

        Date date2 = new Date();
        System.out.println("----程序结束运行----，程序运行时间【" + (date2.getTime() - date1.getTime()) + "毫秒】");
        return new ArrayList<>();







public class ThredQuery implements Callable<Object>
{
    private RedisActiveTestsMapper redisActiveTestsMapper;
    public ThredQuery(RedisActiveTestsMapper redisActiveTestsMapper) {
        this.redisActiveTestsMapper = redisActiveTestsMapper;
    }

    @Override
    public Object call() throws Exception {
        int i=1;
        System.out.println(">>>" + i + "任务启动");
        Date dateTmp1 = new Date();
        Thread.sleep(1000);
        Date dateTmp2 = new Date();
        long time = dateTmp2.getTime() - dateTmp1.getTime();
        List<RedisActiveTests> selectnan = redisActiveTestsMapper.selectnan(new RedisActiveTests());
        System.out.println(">>>" + i + "任务终止");
        i++;
        return JSONArray.toJSONString(selectnan) + "任务返回运行结果,当前任务时间【" + time + "毫秒】";
    }
}

20w相对来说数据量还是很少的，对查询条件加联合索引试试

countdownlatch就能让主线程等待，子线程全部执行完再执行主线程代码！或者jdk8的Future，也能让主线程等待