| From: | Anssi Kääriäinen <anssi(dot)kaariainen(at)thl(dot)fi> |
|---|---|
| To: | Steve Singer <steve(at)ssinger(dot)info> |
| Cc: | Petr Jelinek <petr(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(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>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(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: | 2014-11-10 07:01:06 |
| Message-ID: | 1415602866.8931.127.camel@TTY32 |
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| Lists: | pgsql-hackers pgsql-www |
On Sun, 2014-11-09 at 11:57 -0500, Steve Singer wrote:
> The reason why Jim and myself are asking for the LSN and not just the
> timestamp is that I want to be able to order the transactions. Jim
> pointed out earlier in the thread that just ordering on timestamp allows
> for multiple transactions with the same timestamp.
>
> Maybe we don't need the entire LSN to solve that. If you already have
> the commit timestamp maybe you only need another byte or two of
> granularity to order transactions that are within the same microsecond.
There is no guarantee that a commit with later LSN has a later
timestamp. There are cases where the clock could move significantly
backwards.
A robust solution to storing transaction commit information (including
commit order) in a way that can be referenced from other tables, can be
loaded to another cluster, and survives crashes would be a great
feature. But this feature doesn't have those properties.
- Anssi
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