Re: BUG #18877: PostgreSQL triggers assertion failure

On 2025-Apr-04, Tom Lane wrote:

> However ... it's possible that this isn't the stupidest, most
> brute-force code in Postgres, but I'll wager it's in the top ten.
> Is it really necessary to transform the subquery twice just to
> produce this error?  It seems like the constructed EXPR_SUBLINK
> should produce the same error all by itself.  I tried nuking this
> whole stanza to see whether that would happen, and then there are a
> couple of regression test cases that don't produce the same results.
> But I feel like it could be made to work with a bit more thought.

Ugh, yeah, this is really dumb.  This quick-and-dirty patch (not final
form) gets us halfway there, by checking the targetlist after
transforming the query.  I think this is the behavior we want, although
it's a bit scary that we have so few test cases for this.

Anyway, if we only do this, this query from the regression tests still
doesn't work the way we want:

  SELECT JSON_ARRAY(SELECT FROM (VALUES (1)) foo(i));

It causes an error to be raised _during_ transformation, namely

+ERROR:  table "q" has 0 columns available but 1 columns specified

which we'd like to report differently.  I think we want to verify the
targetlist length of ctor->query before transformation, but I'm not sure
exactly how, yet.

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/