drop table if exists t1,t2,t3;

create table t1 (f1 int unique, f2 text);
insert into t1 values (1, 'one');
create table t2 (f1 int unique, f2 text);
insert into t2 values (2, 'two');
create table t3 (f1 int unique, f2 text);
insert into t3 values (3, 'three');

explain
select * from t1 union all select * from t2 union all select * from t3
order by 1;

begin;

declare c cursor for
select * from t1 union all select * from t2 union all select * from t3
order by 1;

fetch 1 from c;

update t1 set f2 = f2 || ' updated' where current of c;
update t2 set f2 = f2 || ' updated' where current of c;
update t3 set f2 = f2 || ' updated' where current of c;

commit;

table t1;
table t2;
table t3;