Using return value from DELETE for UPDATE in Postgres - postgresql

Question

I need to update a table using a value deleted from another table. The situation is a comment vote scorekeeper similar to the one on SO. I'm using python to work the postgres, but that shouldn't make a difference.
query="""
UPDATE comment SET score=score-(DELETE FROM history
WHERE commentId=%(commentId)s AND
userIdentity=%(userIdentity)s RETURNING vote)
WHERE commentId=%(commentId)s;
"""
cursor.execute(query, data)
The error arises at (DELETE FROM; a syntax error arises. I can replace the DELETE statement with a SELECT statement and it will work, is there something I am missing here? I want to use the returning value in an update. Is this possible? Anything helps.
Relevent schema:
CREATE TABLE history (
commentId bigint,
vote int,
userIdentity varchar(256),
);
CREATE TABLE comment (
id bigint,
score bigint,
);
history.vote is normally 1 or -1.

Answer 1

PostgreSQL doesn't allow mix UPDATE and DELETE statements as subquery.
You can use a little bit different strategy - updateable CTE
postgres=# WITH t1 AS (DELETE FROM foo RETURNING *),
t2 AS (INSERT INTO deleted
SELECT * FROM t1 RETURNING *)
SELECT max(a) FROM t2;
so
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)
Attention: It require 9.1 or newer