O PostgreSQL não permite misturar instruções UPDATE e DELETE como subconsulta.
Você pode usar uma estratégia um pouco diferente - CTE atualizável
postgres=# WITH t1 AS (DELETE FROM foo RETURNING *),
t2 AS (INSERT INTO deleted
SELECT * FROM t1 RETURNING *)
SELECT max(a) FROM t2;
assim
postgres=# CREATE TABLE comment(id int, score int);
CREATE TABLE
postgres=# CREATE TABLE history(id int, comment_id int, vote int);
CREATE TABLE
postgres=# INSERT INTO comment values(1,10);
INSERT 0 1
postgres=# INSERT INTO comment values(2,20);
INSERT 0 1
postgres=# INSERT INTO history values(1,1,5);
INSERT 0 1
postgres=# WITH t1 AS (DELETE FROM history
WHERE id=1
RETURNING comment_id, vote)
UPDATE comment SET score=score-t1.vote
FROM t1
WHERE t1.comment_id=comment.id;
UPDATE 1
postgres=# select * from comment;
id | score
----+-------
2 | 20
1 | 5
(2 rows)
Atenção:Requer 9.1 ou mais recente