From: Andrew Gierth <andrew@tao11.riddles.org.uk>
To: Christoph Berg <myon@debian.org>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
  PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: UCT (Re: pgsql: Update time zone data files to tzdata release
 2019a.)
In-Reply-To: <20190604085735.GD24018@msg.df7cb.de> (Christoph Berg's message
	of "Tue, 4 Jun 2019 10:57:35 +0200")
Message-ID: <87blzdacpp.fsf@news-spur.riddles.org.uk>
References: <E1hK8qL-0005yH-VX@gemulon.postgresql.org>
	<20190604085735.GD24018@msg.df7cb.de>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (berkeley-unix)
Date: Tue, 04 Jun 2019 11:20:14 +0100
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>>>>> "Christoph" == Christoph Berg <myon@debian.org> writes:

 >> Etc/UCT is now a backward-compatibility link to Etc/UTC, instead of
 >> being a separate zone that generates the abbreviation "UCT", which
 >> nowadays is typically a typo. Postgres will still accept "UCT" as an
 >> input zone name, but it won't output it.

 Christoph> There is something wrong here. On Debian Buster/unstable,
 Christoph> using system tzdata (2019a-1), if /etc/timezone is
 Christoph> "Etc/UTC":

 Christoph> 11.3's initdb adds timezone = 'UCT' to postgresql.conf
 Christoph> 12beta1's initdb add timezone = 'Etc/UCT' to postgresql.conf

 Christoph> Is that expected behavior?

It's clearly not what users expect and it's clearly the wrong thing to
do, though it's the expected behavior of the current code:

 * On most systems, we rely on trying to match the observable behavior of
 * the C library's localtime() function.  The database zone that matches
 * furthest into the past is the one to use.  Often there will be several
 * zones with identical rankings (since the IANA database assigns multiple
 * names to many zones).  We break ties arbitrarily by preferring shorter,
 * then alphabetically earlier zone names.

I believe I pointed out a long, long time ago that this tie-breaking
strategy was insane, and that the rule should be to prefer canonical
names and use something else only in the case of a strictly better
match.

If TZ is set or if /etc/localtime is a symlink rather than a hardlink or
copy of the zone file, then PG can get the zone name directly rather
than having to do the comparisons, so the above comment doesn't apply;
that gives you a workaround.

-- 
Andrew (irc:RhodiumToad)