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Date: Mon, 10 Nov 2025 04:21:20 +0100
From: "Kamil Monicz" <kamil@monicz.dev>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: pgsql-bugs@lists.postgresql.org
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In-Reply-To: <1650412.1762703636@sss.pgh.pa.us>
References: <19106-9dd3668a0734cd72@postgresql.org>
 <1650412.1762703636@sss.pgh.pa.us>
Subject: Re: BUG #19106: Potential regression with CTE materialization
 planning in
 Postgres 18
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On Sun, Nov 9, 2025, at 16:53, Tom Lane wrote:
> PG Bug reporting form <noreply@postgresql.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