Re: BUG #17623: WHERE should be evaluated after FROM clause when operators may throw

The following bug has been logged on the website:

Bug reference:      17623
Logged by:          Stamatis Zampetakis
Email address:      zabetak@gmail.com
PostgreSQL version: 14.5
Operating system:   Debian 10.2.1-6
Description:       

Steps to reproduce:
CREATE TABLE emp (empno INT, name VARCHAR, deptno INT);
INSERT INTO emp VALUES (0, 'Alex', 0);
INSERT INTO emp VALUES (10, 'Bob', 1);

CREATE TABLE dept (deptno INT);
INSERT INTO dept VALUES (1);

SELECT e.name
FROM emp e
INNER JOIN dept d ON e.deptno = d.deptno
WHERE (10 / e.empno) = 1

Actual output:
ERROR:  division by zero

Expected output:
Bob

The error is caused since the filter condition in the WHERE clause is
evaluated before the join. Filter push-down is a very common and powerful
optimization but when there are operators in the WHERE clause that may throw
(such as division, cast, etc) this optimization is unsafe.

The SQL standard (Section 7.4 general rule 1) mandates that WHERE should be
applied to the result of FROM so in the case above pushing filters below the
join seems to violate the standard.