Re: tracking commit timestamps

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Steve Singer <steve(at)ssinger(dot)info>
Cc: 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>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Petr Jelinek <petr(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, 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: 2014-11-05 22:43:36
Message-ID: 20141105224336.GB28295@alap3.anarazel.de
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On 2014-11-05 17:17:05 -0500, Steve Singer wrote:
> It isn't just 'replication' systems that have a need for getting the commit
> order of transactions on a single system. I have a application (not slony)
> where we want to query a table but order the output based on the transaction
> commit order of when the insert into the table was done (think of a queue).
> I'm not replicating the output but passing the data to other applications
> for further processing. If I just had the commit timestamp I would need to
> put in some other condition to break ties in a consistent way. I think
> being able to get an ordering by commit LSN is what I really want in this
> case not the timestamp.
>
> Logical decoding is one solution to this (that I was considering) but being
> able to do something like
> select * FROM event_log order by commit_id would be a lot simpler.

Imo that's essentially a different feature. What you essentially would
need here is a 'commit sequence number' - but no timestamps. And
probably to be useful that number has to be 8 bytes in itself.

Greetings,

Andres Freund

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

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