Re: remove_useless_joins vs. bug #19560

These patches fix all my known failure cases (attached.) In addition, as a
plus, a small set of queries that previously didn't compile are now
accepted, for example:

SELECT coalesce(u1.id + 1, 1) AS o0
FROM (u AS u1 FULL JOIN (u AS u2 JOIN u AS u3 ON u2.id = u3.id)
ON u2.id = u2.id)
GROUP BY 1;

... previously gave ERROR: FULL JOIN is only supported with merge-joinable
or hash-joinable join conditions

now returns 10 rows (`2 … 11`), which I believe is correct.

```
HashAggregate
Group Key: COALESCE((u1.id + 1), 1)
-> Merge Full Join
-> Seq Scan on u u1
-> Materialize
-> Seq Scan on u u3
```

So that's nice.

On Sat, Jul 25, 2026 at 7:15 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Here's a patchset addressing that. 0001-0003 are the same as before,
> and then 0004 fixes this bug and adds tests.
>
> Your Claude session mentioned a different failure scenario, but
> gave no examples so I'm not quite sure if this covers it or not.
>
> regards, tom lane
>
>