Re: SSI and Hot Standby

2011/1/21 Anssi Kääriäinen <anssi.kaariainen@thl.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>