How to use read uncommitted transaction level and set update order

How to use column values set in update in subsequent set clauses and in
subqueries in subsequent row updates?

I tried

set transaction isolation level read uncommitted;
create temp table test1 ( a int, b int) on commit drop;
insert into test1 values(1,2);
update test1 set a=4, b=a ;
select * from test1

b value is 1 but must be 4.
How to use updated value ?


For update order I tried

set transaction isolation level read uncommitted;
create temp table test1 ( a int, b int, c int) on commit drop;
insert into test1 values(1,2,3);
update test1 set a=4, b=a order by c ;
select * from test1

but got syntax error at order by.

How to specify update order ?

Andrus.