From: | Nicolas Barbier <nicolas(dot)barbier(at)gmail(dot)com> |
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To: | Anssi Kääriäinen <anssi(dot)kaariainen(at)thl(dot)fi> |
Cc: | "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: SSI and Hot Standby |
Date: | 2011-01-21 13:05:04 |
Message-ID: | AANLkTikL4B7yEQ4WHtj-kxYGsbH0GowHJF3P93G=SVcX@mail.gmail.com |
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2011/1/21 Anssi Kääriäinen <anssi(dot)kaariainen(at)thl(dot)fi>:
> Sorry for bothering all of you, but I just don't get this. What if T2 rolls
> back instead of committing? Then the snapshot of T3 would have been valid,
> right? Now, for the snapshot of T3 it doesn't matter if T2 commits or if it
> doesn't, because it can't see the changes of T2 in any case. Thus, it would
> seem that the snapshot is valid. On the other hand I can't see anything
> wrong in the logic in your post. What am I missing? I am feeling stupid...
>
> At least for dumps I don't see how T2 can matter (assuming T3 is the
> pg_dump's snapshot). Because if you reload from the dump, T2 never happened
> in that dump. In the reloaded database it just did not exist at all.
This has been discussed before; in [1] I summarized:
"IOW, one could say that the backup is consistent only if it were
never compared against the system as it continued running after the
dump took place."
Nicolas
[1] <URL:http://archives.postgresql.org/pgsql-hackers/2010-09/msg01763.php>
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