PostgreSQL - Unexpected division by zero using SUM

Question

This query (minimal reproducible example):

WITH t as (
    SELECT 3 id, 2 price, 0 amount
)
SELECT 
    CASE WHEN amount > 0 THEN
        SUM(price / amount)
    ELSE
        price
    END u_price
FROM t
GROUP BY id, price, amount

on PostgreSQL 9.4 throws

division by zero

Without the SUM it works.

How is this possible?

Answer 1

I liked this question and I turned for help to these tough guys :

The planner is guilty:

A CASE cannot prevent evaluation of an aggregate expression contained within it, because aggregate expressions are computed before other expressions in a SELECT list or HAVING clause are considered

More details at https://www.postgresql.org/docs/10/static/sql-expressions.html#SYNTAX-EXPRESS-EVAL