begin;
seb=> create table tab (tab_pk serial primary key, x text, t int);
seb=> select * from tab;
 tab_pk | x | t
--------+---+---
       1  | b |
       2  | c |
       3  | a |
( 3  rows)
seb=> create temp sequence g;

seb=> update tab set t = x.g from (
  select *, nextval('g') as g from (
    select * from tab order by x
  ) as x
) as x where tab.tab_pk = x.tab_pk;

seb=> select * from tab;
 tab_pk | x | t
--------+---+---
       3  | a |  1 
       1  | b |  2 
       2  | c |  3 
( 3  rows)
seb=> EXPLAIN update tab set t = x.g from (select *, nextval('g') as g from (select * from tab order by x) as x) as x where tab.tab_pk = x.tab_pk;
                                QUERY PLAN
--------------------------------------------------------------------------
 Hash Join  (cost= 116 . 74 .. 164 . 59  rows= 1160  width= 50 )
   Hash Cond: (x.tab_pk = tt.tab.tab_pk)
   ->  Subquery Scan x  (cost= 80 . 64 .. 98 . 04  rows= 1160  width= 40 )
         ->  Sort  (cost= 80 . 64 .. 83 . 54  rows= 1160  width= 40 )
               Sort Key: tt.tab.x
               ->  Seq Scan on tab  (cost= 0 . 00 .. 21 . 60  rows= 1160  width= 40 )
   ->  Hash  (cost= 21 . 60 .. 21 . 60  rows= 1160  width= 42 )
         ->  Seq Scan on tab  (cost= 0 . 00 .. 21 . 60  rows= 1160  width= 42 )
( 8  rows)
commit;