Re: BUG #19106: Potential regression with CTE materialization planning in Postgres 18

On Sun, Nov 9, 2025, at 16:53, Tom Lane wrote:
> PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> > After upgrading from Postgres 17 to 18, one of my queries started raising an
> > error:
> > "unexpected outer reference in CTE query"
>
> I agree that sounds like a bug ...
>
> > The problematic query is:
> > https://github.com/openstreetmap-ng/openstreetmap-ng/blob/eb805d8766fb4b359b96eb6b50acc8c2a835a165/app/services/element_spatial_service.py#L82-L215
>
> ... but I am not going to spend time trying to reproduce it given
> this amount of detail. There's too much missing context, like what
> data you were running the query on. I could spend all day, not
> see the failure, and be left no wiser than before as to whether
> it's already fixed or I just didn't duplicate your context closely
> enough. Please see if you can reduce the problem case to a
> self-contained SQL script.
>
> regards, tom lane
>

Hello Tom,

It's my first time here (and realistically on a proper mailing list), so please excuse me. Here's the small, self-contained reproduction:

```
EXPLAIN
SELECT *
FROM (
SELECT ARRAY[1, 2] AS arr
) r
CROSS JOIN LATERAL (
WITH a AS (
SELECT
CASE
WHEN id = 1 THEN ST_GeomFromText('LINESTRING(0 0,1 0,1 1)')
ELSE ST_GeomFromText('POINT(0 0)')
END AS geom
FROM unnest(r.arr) AS id
),
b AS (
SELECT ST_Polygonize(
(SELECT ST_UnaryUnion(ST_Collect(geom)) FROM a)
) AS st_polygonize
)
SELECT
(SELECT st_polygonize FROM b),
(SELECT st_polygonize FROM b)
) s;
```
```
ERROR: unexpected outer reference in CTE query

SQL state: XX000
```

It depends on PostGIS being installed and loaded. In my case, it's version 3.6.0. I tried to make it work without it, but I couldn't figure it out.

-Kamil Monicz