Luke McFarlane <luke@fisheye.com.au> writes:
> It's interesting why addition assumes one operand is interval while
> subtraction assumes it is timestamp? It should at least be consistent, IMHO.
There is no timestamp + timestamp operator, for reasons that should be
fairly apparent. There are both timestamp - timestamp (yielding interval)
and timestamp - interval (yielding timestamp), and in case of ambiguity
the preferred interpretation is the one with the same datatype on both
sides. See
http://www.postgresql.org/docs/7.4/static/typeconv-oper.html
regards, tom lane