PostgreSQL schema import that contains a multi-SELECT VIEW

Hi! Again, a very nieche problem. VIEWs, that look like this:

```sql
CREATE VIEW public.... AS
SELECT ...
FROM (...
JOIN ... ON ((... = ...)))
UNION
SELECT ...
FROM (...
JOIN ... ON ((... = ...)));
```

cannot be loaded, due to the second SELECT statement after the UNION. Again, removing this just works fine.

Reproduce the problem by importing a PostgreSQL schema containing a VIEW with multiple SELECT statements joined by UNION. Locate the schema-import entry point and its existing import tests, then add coverage for this view shape. Done means the schema imports successfully without removing the second SELECT.

Written by the indexing model from the issue text.