Reported type mismatch improperly

Currently when we call select_common_type, it compares the 2 exprs, if the
expr
type of both are unknown, it will be set to TEXTOID with some reasons, which
can cause the issue like below.

postgres=# select null union all select null union all select 1;
ERROR: UNION types text and integer cannot be matched
LINE 1: select null union all select null union all select 1;

In this case, we can't blame the user, they may want the nulls to be at
the top
of the result.

I worked on a patch to fix this, the main idea is before going to the above
logic, I peak all the exprs for a given column first, and choose a default
one
when we see the Unknown & Unknown case(rather than TextOid),

do you think it is ok?

--
Best Regards
Andy Fan