Re: [HACKERS] Cutting initdb's runtime (Perl question embedded)

Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > On Tue, May 8, 2018 at 4:48 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> Really the only thing here that jumps out as being unduly expensive for
> >> what it's doing is select_default_timezone.  That is, and always has been,
> >> a brute-force algorithm; I wonder if there's a way to do better?
> 
> > Who says we need a portable way?  If we had something that worked on
> > Linux and macOS, it would cover most developer environments.  I wonder
> > if readlink("/etc/localtime", buf, sz) might be a viable approach.
> 
> I wondered about that, but I'm afraid it's often a hardlink not a
> symlink.  Still, we could try it.

In Debian systems, it's a symlink.  Apparently in RHEL6 and older it's a
copy or hardlink, and the file /etc/sysconfig/clock contains a ZONE
variable that points to the right zone.  Maybe if we add enough
platform-dependent hacks, we would use the slow fallback only for rare
cases.  (Maybe have initdb emit a warning when the fallback is used, so
that we know what else to look for.)

This comment is insightful:
https://github.com/HowardHinnant/date/issues/269#issuecomment-353792132

It's talking about this code:
https://github.com/HowardHinnant/date/blob/master/src/tz.cpp#L3652

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