Postgres and fs quotas

I am trying to set up a server with quotas, and it occured to me that
all the DB files would be owned by postgres (I think). Is there a way to
make the quotas count a user's DB toward that user instead of postgres?
Are there any reccommendations to this end? I am not entirely familiar
with quotas, I assumed that quotas count all files owned by a user (or
group, as the case may be).

Thanks,
    Jeff Davis

PS: Thanks for the last tip (Tom Lane), that worked exactly like I
wanted.