Re: BUG #16396: Parallel Union queries seem to treat NULL values differently

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Monday, April 27, 2020, PG Bug reporting form <noreply(at)postgresql(dot)org>
> wrote:
>> But as in our query the following happened:
>> SELECT hello_world
>> FROM (SELECT CAST(NULL AS INTEGER) AS hello_world) as a
>> UNION
>> SELECT hello_world
>> FROM (SELECT NULL AS hello_world) as b
>> It fails and is only fixable by defining the NULL in the right side of the
>> UNION as CAST(NULL AS INTEGER.

> In the example the second null is untyped still when it gets fed to the
> used and so can be implicitly cast to match the left side of the union. In
> you real case the null appears in a subquery under the from clause and
> needs to be made into an actual type before it can be passed out of the
> subquery and appear in the main query. The resultant type here is text.
> Then the union happens and integer and text cannot be melded together.

Yeah. I believe this changed in v10, at this commit:

https://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=1e7c4bb0049732ece651d993d03bb6772e5d281a

Probably that should have been called out as an incompatibility in the
v10 release notes, but I don't immediately see anything there that
matches. In any case, it's intentional.

regards, tom lane