BUG #18091: Unexpected Result by enable_material

The following bug has been logged on the website:

Bug reference:      18091
Logged by:          Jinsheng Ba
Email address:      bajinsheng@u.nus.edu
PostgreSQL version: 15.4
Operating system:   Ubuntu
Description:

Hello,

The same query returns different results when enable_material is changed:

CREATE TABLE t2(c1 DECIMAL, c2 serial);
CREATE TABLE t4(c0 TEXT);
INSERT INTO t2(c2) VALUES(1), (2);
INSERT INTO t2(c1) VALUES(0.1);
INSERT INTO t4(c0) VALUES('a'), ('b'), ('c');

SET enable_material=true;
SELECT DISTINCT ON (t2.c1) t2.c2 FROM t2, t4; -- {1}, {1}
SET enable_material=false;
SELECT DISTINCT ON (t2.c1) t2.c2 FROM t2, t4; -- {1}, {2}


Reproduction:
docker run -p 5432:5432 -e POSTGRES_PASSWORD=root -it postgres:latest
psql "postgresql://postgres:root@localhost:5432"