Re: Assignment of valid collation for SET operations on queries with UNKNOWN types.

I wrote:
> Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> writes:
>> UNKNOWN is not exactly a pseudo-type.

> Well, as I said to Michael just now, I think we should turn it into one
> now that we're disallowing it in tables, because "cannot be used as a
> table column" is more or less the definition of a pseudotype.

I experimented with this, and it actually doesn't seem to be any harder
than the attached: there's one type_sanity query that changes results,
and otherwise all the regression tests pass.

I've grepped the code for references to UNKNOWNOID and TYPTYPE_PSEUDO,
and I can't find any places where the behavior would change in a way
that we don't want. Basically it looks like we'd disallow UNKNOWN as
a domain base, a PL function argument or result, and a plpgsql local
variable; and all of those seem like good things from here.

Have not checked the docs.

regards, tom lane