Re: BUG #18091: Unexpected Result by enable_material

On Thu, Sep 7, 2023 at 3:10 PM PG Bug reporting form <noreply(at)postgresql(dot)org>
wrote:

> 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}

With DISTINCT ON, only the first row of each set is kept, which is
unpredictable. Usually you're supposed to use ORDER BY to get the
desired row.

Thanks
Richard