From: Tom Lane <tgl@sss.pgh.pa.us>
To: exclusion@gmail.com
cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18536: Using WITH inside WITH RECURSIVE triggers a
 "shouldn't happen" error
In-reply-to: <18536-0a342ec07901203e@postgresql.org>
References: <18536-0a342ec07901203e@postgresql.org>
Comments: In-reply-to PG Bug reporting form <noreply@postgresql.org>
	message dated "Sat, 13 Jul 2024 09:00:00 -0000"
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Date: Sun, 14 Jul 2024 11:09:37 -0400
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PG Bug reporting form <noreply@postgresql.org> writes:
> The following query:
> WITH RECURSIVE t(n) AS (
>     WITH t1 AS (SELECT 1 FROM t) SELECT 1
>     UNION
>     SELECT 1 FROM t1)
> SELECT * FROM t;

That should throw an error, certainly: it's not a valid recursive
structure.  (Since the inner WITH clause spans the whole
"SELECT 1 UNION SELECT 1 FROM t1" structure, we don't have a top-
level UNION anymore.)  But it shouldn't throw this error:

> ERROR:  XX000: missing recursive reference
> LOCATION:  checkWellFormedRecursion, parse_cte.c:896

We do get the right behaviors for WITHs that are down inside one
side or the other of the UNION:

WITH RECURSIVE t(n) AS (
    (WITH t1 AS (SELECT 1 FROM t) SELECT 1 FROM t1)
    UNION
    SELECT 1)   =

SELECT * FROM t;
ERROR:  recursive reference to query "t" must not appear within its non-re=
cursive term
LINE 2:     (WITH t1 AS (SELECT 1 FROM t) SELECT 1 FROM t1)
                                       ^

WITH RECURSIVE t(n) AS (
  SELECT 1
  UNION
  (WITH t1 AS (SELECT 1 FROM t) SELECT 1 FROM t1))
SELECT * FROM t;
 n =

---
 1
(1 row)

I think the case you show should be throwing

ERROR:  recursive query "t" does not have the form non-recursive-term UNIO=
N [ALL] recursive-term

Will look closer later.  Thanks for the report.

			regards, tom lane