BUG #18091: Unexpected Result by enable_material

On Thu, 7 Sept 2023 at 09:10, PG Bug reporting form
<noreply@postgresql.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.

On Thu, Sep 7, 2023 at 3:10 PM PG Bug reporting form <noreply@postgresql.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