Re: BUG #15129: Problem with UNION/UNION ALL type setting when several NULL values before defining the proper type

Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> "PG" == PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
> PG> While migrating a view from another RDBMS,I reached something that
> PG> looks lie a bug in postgres.

> It's not a bug in the code, though perhaps you could point out a place
> where the documentation could be improved?

It strikes me that section 10.5 doesn't say explicitly that multiple
UNIONs are resolved pairwise. Someone who expected "x union y union z"
to be resolved holistically, like a 3-way CASE would be, is not going
to be enlightened by that section. Perhaps an additional example
using this exact situation would be helpful.

> (The SQL standard is of no particular help here since it does not allow
> NULL to appear "bare", except in contextually typed expressions.)

I think the SQL spec does offer considerable support for our pairwise
resolution approach --- nowhere does it suggest that "x union y union z"
is a construct that should be considered as a whole rather than as a
nest of two independent union operations.

regards, tom lane