Re: BUG #18091: Unexpected Result by enable_material

On Thu, 7 Sept 2023 at 09:10, PG Bug reporting form
<noreply(at)postgresql(dot)org> wrote:
> The same query returns different results when enable_material is changed:

I'm not too sure your query is guaranteed to return the same result
allways, let me see..

-- Zapping t4....
> CREATE TABLE t2(c1 DECIMAL, c2 serial);
> INSERT INTO t2(c2) VALUES(1), (2);
> INSERT INTO t2(c1) VALUES(0.1);
This seems to leave t2 as (null,1),(null,2),(0.1,1)
--
...
> SELECT DISTINCT ON (t2.c1) t2.c2 FROM t2, t4; -- {1}, {1}
> SELECT DISTINCT ON (t2.c1) t2.c2 FROM t2, t4; -- {1}, {2}

And this seems to ask for one row from
(null,1,{a,b,c})+(null,2,{a,b,c}), another from (0.1,1,{a,b,c})

So your result would be (1|2) + 1 ( in any order ), as the first set
can be scanned/selected in any order ( and the results returned in any
order )

Normally postgres would not bother on generating different outputs if
nothing is changed, but I think any response in {1,1},{1,2},{2,1} is
correct. Enable_material is probably just changing some access order
details.

You may be able to get better repeatable results using order by judiciously.

Francisco Olarte.