Robert Haas <[email protected]> writes:
> On Thu, Jun 16, 2011 at 6:54 PM, Tom Lane <[email protected]> wrote:
>> 4. Backend #2 visits the new, about-to-be-committed version of
>> pgbench_accounts' pg_class row just before backend #3 commits.
>> It sees the row as not good and keeps scanning. By the time it
>> reaches the previous version of the row, however, backend #3
>> *has* committed. So that version isn't good according to SnapshotNow
>> either.
> <thinks some more>
> Why isn't this a danger for every pg_class update? For example, it
> would seem that if VACUUM updates relpages/reltuples, it would be
> prone to this same hazard.
VACUUM does that with an in-place, nontransactional update. But yes,
this is a risk for every transactional catalog update.
regards, tom lane
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