批量删除区别

delete() 和 deleteInBatch()
deleteAll() 和 deleteAllInBatch()

普通删除方法会根据唯一标识字段，生成多条 SQL 语句，而有 InBatch 的方法只会生成一条 SQL 语句

写入大量数据

@Transactional
public void save100WEntities() {
    for (int i = 0; i < 1000; i++) {
        respsitory.save(1000Entities);

         entityManager.flush();
        entityManager.clear();
    }
}

读取大量数据

需要满足3个条件：

1. ResultSetType 是 ResultSet.TYPE_FORWARD_ONLY
2. ResultSetConcurrency 是 ResultSet.CONCUR_READ_ONLY
3. FetchSize 是 Integer.MIN_VALUE

Statement statement = connection.createStatement(
        ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
statement.setFetchSize(Integer.MIN_VALUE);

@Repository
public interface UserRepository extends JpaRepository<User, Integer> {
    @QueryHints(value = @QueryHint(name = FETCH_SIZE, value = "" + Integer.MIN_VALUE))
    Stream<User> findByIdIsNotNull();
}

@Transactional
public void run() {
    try (Stream<User> stream = userRepository.findByIdIsNotNull()) {
        stream.forEach(user -> {
            System.out.println(user);
            entityManager.detach(user);
        });
    }
}

场景：当 users 表在 insert，update，delete 时，在 users_log 表中记录变更的 id

MySQL

触发时机：BEFORE，AFTER
触发事件：INSERT，UPDATE，DELETE
NEW 和 OLD 关键字：

| action | NEW | OLD |
|--------|----------------|----------------|
| insert | 插入的新数据 | 无 |
| update | 修改为的新数据 | 被修改的原数据 |
| delete | 无 | 被删除的原数据 |

DELIMITER $
CREATE TRIGGER insert_user AFTER INSERT
ON users FOR EACH ROW 
BEGIN
    INSERT INTO users_log(user_id, action) VALUES(NEW.id, 'insert');
END$
DELIMITER ;

前提条件

public interface Animal {
    void say();

    void run(String addr);
}

public class Dog implements Animal {
    @Override
    public void say() {
        System.out.println("汪汪汪");
    }

    @Override
    public void run(String addr) {
        say();
        System.out.println("狗在" + addr + "跑");
    }
}