No-rewrite timestamp<->timestamptz conversions

On Tue, Nov 05, 2013 at 05:02:58PM -0800, Josh Berkus wrote:
> I'd also love some way of doing a no-rewrite conversion between
> timestamp and timestamptz, based on the assumption that the original
> values are UTC time.  That's one I encounter a lot.

It was such a conversion that motivated me to add the no-rewrite ALTER TABLE
ALTER TYPE support in the first place.  Interesting.  Support for it didn't
end up in any submitted patch due to a formal problem: a protransform function
shall only consult IMMUTABLE facts, but we posit that timezone==UTC is a
STABLE observation.  However, a protransform function can easily simplify the
immutable expression "tscol AT TIME ZONE 'UTC'", avoiding a rewrite.  See
attached patch.  Examples:

begin;
create table t (c timestamptz);
set client_min_messages = debug1;
-- rewrite: depends on timezone GUC
alter table t alter c type timestamp;
-- rewrite: depends on timezone GUC
alter table t alter c type timestamptz;
-- no rewrite: always UTC+0
alter table t alter c type timestamp using c at time zone 'UTC';
-- no rewrite: always UTC+0
alter table t alter c type timestamptz using c at time zone 'Etc/Universal';
-- rewrite: always UTC+0 in the present day, but not historically
alter table t alter c type timestamp using c at time zone 'Atlantic/Reykjavik';
-- rewrite: always UTC+0 in the present day, but not historically
alter table t alter c type timestamptz using c at time zone 'Africa/Lome';
-- no rewrite: always UTC+0
alter table t alter c type timestamp using c at time zone 'GMT';
-- rewrite: always UTC+1
alter table t alter c type timestamptz using c at time zone '1 hour'::interval;
-- no rewrite: always UTC+0
alter table t alter c type timestamp using c at time zone '0 hour'::interval;
rollback;