Re: tracking commit timestamps

From: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Steve Singer <steve(at)ssinger(dot)info>, 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: 2014-12-04 04:11:05
Message-ID: CAHGQGwGGbS4ODS2vEF6A1qERPzF_=_MOQA1DmmW7j5z9q75cGQ@mail.gmail.com
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On Thu, Dec 4, 2014 at 12:58 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 4 December 2014 at 03:08, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
>> #1. set up and start the master and standby servers with
>> track_commit_timestamp disabled
>> #2. enable track_commit_timestamp in the master and restart the master
>> #3. run some write transactions
>> #4. enable track_commit_timestamp in the standby and restart the standby
>> #5. execute "select pg_xact_commit_timestamp('1000'::xid)" in the standby
>
> I'm not sure what step4 is supposed to do?
>
> Surely if steps 1-3 generate any WAL then the standby should replay
> it, whether or not track_commit_timestamp is enabled.
>
> So what effect does setting that parameter on the standby?

At least track_commit_timestamp seems to need to be enabled even in the standby
when we want to call pg_xact_commit_timestamp() and pg_last_committed_xact()
in the standby. I'm not sure if this is good design, though.

Regards,

--
Fujii Masao

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