Hi Roman

> > 4. Heterogeneous queries to external data source
> >
> > Table qualifier: <ds_table_name>@<ds_name>
>
> I'd suggest to extend the tables like this:
>
> CREATE TABLE table [EXTERNAL {[FILE] filespec | DATA SOURCE <ds_name>}]

Your variant means that names of tables in both database will be identical. It is not always comprehensible. Vlad's variant can be solve next use case:
select ... from Table1@DS1 join Table1@DS2 on ...

Also change of a name of the table in external database will demand a re-registration of this table. It is more movements, probability of an error hence there is more.

WBR Evgeny Putilin