Simon Riggs <simon@2ndquadrant.com> writes:
> On Mon, 2007-11-19 at 10:38 -0500, Tom Lane wrote:
>> The race conditions are a lot more subtle than that. The stats
>> collector cannot know when it receives a tabstat message after VACUUM
>> starts whether VACUUM has/will see the tuples involved, or whether it
>> will see them as committed or not. That would depend on whether VACUUM
>> has yet reached the page(s) the tuples are in.
> I think the before-and-after approach can be made to work:
> VACUUM just needs to save the counter in memory, it doesn't need to
> write that anywhere else.
> VACUUM can force the flush of the tabstat file so that there is no race
> condition, or at least a minimised one.
I don't think you understood what I said at all. The race condition is
not "before vs after VACUUM starts", it is "before vs after when VACUUM
scans the page that the in-doubt tuple is in".
regards, tom lane