Interval+tz-tz gives unexpected result

"Henshall, Stuart - Design & Print" <SHenshall@westcountry-design-print.co.uk> writes:

dev=# select '0 sec'::interval+'2002/10/21 09:48'::timestamptz-'2002/10/21
09:30'::timestamptz;
?column?
----------
-09:30
(1 row)

In 7.3 I get

regression=# select '0 sec'::interval+'2002/10/21 09:48'::timestamptz-'2002/10/21 09:30'::timestamptz;
ERROR: Unable to identify an operator '+' for types 'interval' and 'timestamp with time zone'
You will have to retype this query using an explicit cast

I'm not sure what interpretation 7.2 is putting on this, but I see that
the addition is yielding a timestamp without time zone:

regression=# select '0 sec'::interval+'2002/10/21 09:48'::timestamptz;
?column?
---------------------
2002-10-21 00:00:00
(1 row)

I *think* it may be coercing the inputs so it can use the date + time
without timezone operator. Which is a tad silly. We've tightened up
the implicit coercions for 7.3, which is why it doesn't do this anymore.

In the meantime, the real issue for you is that there's a timestamptz +
interval operator, but not interval + timestamptz, so you need to recast
the command as timestamptz + interval - timestamptz.

regards, tom lane