Re: Unsafe qual pushdown through DISTINCT with simple CASE expressions

On Fri, Aug 28, 2026 at 4:15 PM Ewan Young <kdbase(dot)hack(at)gmail(dot)com> wrote:
> CREATE COLLATION ci (provider=icu, locale='und-u-ks-level2',
> deterministic=false);
> CREATE TABLE cit (t text COLLATE ci);
> INSERT INTO cit VALUES ('a'),('A'),('b');
>
> -- pushed on master, but v2 keeps the Filter above the grouping
> EXPLAIN (COSTS OFF)
> SELECT * FROM (SELECT DISTINCT t FROM cit) d
> WHERE (CASE t WHEN JSON_OBJECT('a': 'b' RETURNING text)
> THEN 1 ELSE 0 END) = 1;

Nice catch. I did not realize CaseTestExpr can be used in a JSON
constructor's coercion expression. I've fixed that and then pushed
the patch.

(Interestingly, eval_const_expressions also mishandles CaseTestExpr in
a JSON constructor's coercion expression, and thus has the same bug.
See [1].)

[1] https://postgr.es/m/CAMbWs48A=VCFbteTkuCoknO1_0-Cu0aMBT0M07dm7vj1QyixDg@mail.gmail.com

- Richard