update order

hello!

i am migrating from mysql to postgre and i have one little problem. i
would like you to help me the best way to solve this pain...

so in mysql i had superious query:
select @i:=0;
update MyTable set OrderCol=(@i:=(at)i+1) order by OrderCol asc;

but in postgre is no way to update and order in same query. alos there
is no user defined variables, bet i make one more sequence.

to resolve this problem i have to make selection ant then each resulting
row with make a update. this is damn much queries! how to make it not so
complex?

may be you can suggest me the best way to resolve this liitle problem?

thank you!