"Michael Richards" <michael(at)fastmail(dot)ca> writes:
> The reduction of the list doesn't seem to be terribly efficient. Here
> are some strategies I've been looking at:
> select id from users WHERE
> id not in (select userid from sentletters where lettertype=1) AND
> aclgroup IN (1,2);
Try an EXCEPT, along the lines of
(select id from users where conditions) except
(select userid from sentletters where other-conditions);
regards, tom lane