Re: tracking commit timestamps

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, Steve Singer <steve(at)ssinger(dot)info>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Anssi Kääriäinen <anssi(dot)kaariainen(at)thl(dot)fi>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Jaime Casanova <jaime(at)2ndquadrant(dot)com>
Subject: Re: tracking commit timestamps
Date: 2014-11-25 15:23:22
Message-ID: 20141125152322.GG1639@alvin.alvh.no-ip.org
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Robert Haas wrote:
> On Tue, Nov 25, 2014 at 9:19 AM, Alvaro Herrera
> <alvherre(at)2ndquadrant(dot)com> wrote:
> > Fujii Masao wrote:
> >> On Tue, Nov 25, 2014 at 7:58 AM, Alvaro Herrera
> >> <alvherre(at)2ndquadrant(dot)com> wrote:
> >> >> And here is v10 which fixes conflicts with Heikki's WAL API changes (no
> >> >> changes otherwise).
> >> >
> >> > After some slight additional changes, here's v11, which I intend to
> >> > commit early tomorrow. The main change is moving the test module from
> >> > contrib to src/test/modules.
> >>
> >> When I specify the XID of the aborted transaction in pg_xact_commit_timestamp(),
> >> it always returns 2000-01-01 09:00:00+09. Is this intentional?
> >
> > Well, when a transaction has not committed, nothing is written so on
> > reading we get all zeroes which corresponds to the timestamp you give.
> > So yeah, it is intentional. We could alternatively check pg_clog and
> > raise an error if the transaction is not marked either COMMITTED or
> > SUBCOMMITTED, but I'm not real sure there's much point.
>
> Maybe 0 should get translated to a NULL return, instead of a bogus timestamp.

That's one idea --- surely no transaction is going to commit at 00:00:00
on 2000-01-01 anymore. Yet this is somewhat discomforting.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

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