recreate table foo(id int primary key);
commit;
set term ^;
execute block as
begin
  execute statement 'drop sequence g'; when any do begin end
end
^ set term ;^
commit;
create sequence g;
commit;

insert into foo values(1);
insert into foo values(2);
insert into foo values(3);
commit;

alter table foo add constraint foo_bar
check(
     exists(select * from foo x where gen_id(g,1) >= 0 and x.id > 0  and x.id <> foo.id ) 
    );

set count on;
set echo on;

update foo set id = -id; -- должен обломаться или нет ? 

set count off;

select * from foo;
select gen_id(g, 0) from rdb$database;

update foo set id = -id;
Records affected: 3

set count off;

select * from foo;

          ID
============
          -1
          -2
          -3

select gen_id(g, 0) from rdb$database;

               GEN_ID
=====================
                    3

update foo set id = -id;
Records affected: 3

set count off;

select * from foo;

          ID
============
          -1
          -2
          -3

select gen_id(g, 0) from rdb$database;

               GEN_ID
=====================
                    6