sql - PostgreSQL AVG函数中的关键字ALL

Question

假设我有一个员工表，其中包含他们的薪水。有什么区别:

SELECT AVG(salary)   
FROM employee; 

和

SELECT AVG(ALL salary)    
FROM employee; 

ALL 是做什么的？两种情况都给出相同的结果。

Answer 1

根据documentation无论聚合函数如何，它们都完全相同:

The first form of aggregate expression invokes the aggregate once for each input row. The second form is the same as the first, since ALL is the default.