Postgres 10 problem with UNION ALL of null value in "subselect"

[Martin Swiech]

Hi folks,

I got some complex query which works on PostgreSQL 9.6 , but fails on
PostgreSQL 10.

Version of PostgreSQL:
PostgreSQL 10.3 on x86_64-apple-darwin14.5.0, compiled by Apple LLVM
version 7.0.0 (clang-700.1.76), 64-bit

Simplified core of the problematic query looks like this:
```
select * from (
   select 1::integer as a
) t1
union all
select * from (
   select null as a
) t2;
```

It fails with this error message:
```
ERROR:  UNION types integer and text cannot be matched
LINE 5: select * from (
               ^
SQL state: 42804
Character: 66
```


It worked on PostgreSQL 9.6.


Query without wrapping subselects (t1 and t2) works on both versions of
PostgreSQL (9.6 and 10) well:
```
select 1::integer as a
union all
select null as a;
```


Is there some new optimization of query processing in PostgreSQL 10, which
needs some "early type determination", but named subselects (t1 and t2)
shades the type from first query?

Or could it be some regression bug?

Thanks for answer.

Martin Swiech