3-d dialect base.

CREATE TABLE TEST_TABLE1
(ID BIGINT,
 FK_ID INTEGER,
 REG_DATE TIMESTAMP NOT NULL);

COMMIT;

insert into test_table1 values (1,5,'01.01.2000');
insert into test_table1 values (2,5,'01.01.2001');
insert into test_table1 values (3,7,'01.01.2002');
insert into test_table1 values (4,8,'01.01.2003');
insert into test_table1 values (5,8,'01.01.2004');
insert into test_table1 values (6,8,'01.01.2005');
insert into test_table1 values (7,8,'01.01.2007');

COMMIT;

Now

select t.fk_id,(select first 1 t1.reg_date from test_table1 t1 where t1.fk_id = t.fk_id
                order by min(t.fk_id))
from test_table1 t
group by t.fk_id

returns error (2.1 RC2)

Unsuccessful execution caused by a system error that precludes
successful execution of subsequent statements
internal gds software consistency check (EVL_expr: invalid operation (232), file: evl.cpp line 1164)

Best regards, Eugene