Re: pg_system_identifier()

Christopher Browne <cbbrowne@gmail.com> writes:
> On Fri, Aug 23, 2013 at 3:10 AM, Fabien COELHO <coelho@cri.ensmp.fr> wrote:
>> Would it make sense for such identifiers be standard UUID
>> (http://en.wikipedia.org/wiki/UUID)?

> There is sense to this, sure.

That ship's already sailed, though.  As was pointed out upthread, we don't
really want to change the way that pg_controldata prints the system ID,
and we don't want this SQL function printing something different either.

> I'd think that constructing a Type 5 (SHA-1) UUID based on some local
> information would make a lot of sense.

> In effect, based on constructing SHA-1 on a string looking like:

> "Database system identifier:           5651554613500795646
> Maximum data alignment:               8
> Database block size:                  8192
> WAL block size:                       8192
> Maximum length of identifiers:        64
> Date/time type storage:               64-bit integers
> Version:  PostgreSQL 9.1.1 on x86_64-unknown-linux-gnu, compiled by
> gcc (Debian 4.6.1-4) 4.6.1, 64-bit"

> ==> SHA-1 of b1b012cc85149d2fe4bf0fc18c38dcf1218e95a5

Including the version string would be a seriously bad idea --- you
don't want the sys ID to change just because you did a minor version
upgrade, or even recompiled the same version with a newer compiler,
do you?  There might be some point in factoring in those other values,
but I'm not terribly excited about them either.
        regards, tom lane