Thread: 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"

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.