I'm using PostgreSQL and NHibernate 1.0.2 and I have to use following HQL:

SELECT EXTRACT(EPOCH FROM sum(x.End - x.Start))
FROM TASK AS x
WHERE x.ID=5

I get a QueryException saying "undefined alias or unknown mapping: EXTRACT". EXTRACT is a function in Postgres so its clear why it is saying that but what can I do to solve this?

Any suggestions?

Is that query a sub query?
if not why don't you just query for x and then do the computation in java?

That's what I did but with PostgreSQL doing the work it would be faster... but not enough to effort more time getting this to work. So I'll keep this solution. Thank you!