Subject Re: [IBO] Problem with Firebird 3
Author Svein Erling Tysvær
Could it be that IBO expects there to be an alias? Have you tried something like:

select t1.field1, (select t2.field1 from table2 t2 where t2.table1_id = t1.table1_id)
from table1 t1

It's just a guess, I've no clue whether or not it works, most of us have primary keys for our tables (and with a primary key included in the query, I'm pretty certain IBO - provided KeyLinksAutoDefine is true - uses this primary key and not RDB$DB_KEY).