如何在PostgreSQL中动态创建单调递增的序列？

Question

我有两个域实体：雇员和部门。 DDL看起来像这样：

CREATE TABLE department(
  id SERIAL PRIMARY KEY
);

CREATE TABLE employee(
  id SERIAL PRIMARY KEY,
  department_id integer,
  index_in_department integer
);

因此，主要字段是 index_in_department 。每个部门的数量都有独特的单调递增。

例如，我有2个部门和6名员工：

department - id == 1
department - id == 2

employee - id == 1, department_id == 1, index_in_department == 1
employee - id == 2, department_id == 1, index_in_department == 2
employee - id == 3, department_id == 1, index_in_department == 3
employee - id == 4, department_id == 2, index_in_department == 1
employee - id == 5, department_id == 2, index_in_department == 2
employee - id == 6, department_id == 2, index_in_department == 3

有什么方法可以在PostgreSQL中自动生成 index_in_department ？

由于多个线程并发插入，因此无法在应用程序级别上生成它。我知道，我可以使用数据库锁（例如咨询锁），但我想要更简单，更友好的解决方案。