Re: BUG #18594: CASE WHEN ELSE failing to return the expected output when the same colum is used in WHEN and ELSE

In the 6 rules for type resolution, rule 5 says "select the first
non-unknown input type as candidate, and consider if other non-unknown
types can be implicitly converted." ... should there not be a new rule,
something like considering if the unknown types can be converted, then
notice that a long string can't be converted to a char, because it won't
fit? And then fail with an interesting error?


On Thu, 29 Aug 2024 at 08:11, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> =?UTF-8?Q?Francisco_J=2E_Ossand=C3=B3n?= <fco.j.ossandon@gmail.com>
> writes:
> > So is the ELSE column hijacking the data type of the whole expression?
>
> It's the only CASE result that is supplying a definite type at all.
> But see
>
> https://www.postgresql.org/docs/current/typeconv-union-case.html
>
> particularly the footnote to the bit about "Select the first
> non-unknown input type as the candidate type, then consider
> each other non-unknown input type, left to right."
>
> The WHEN clauses have exactly nothing to do with the result type
> of the CASE: it's the THEN and ELSE clauses that supply the result.
>
>                         regards, tom lane
>
>
>