Re: Autovacuuming

Jim C. Nasby wrote:
> On Mon, Apr 24, 2006 at 08:41:55AM -0400, Alvaro Herrera wrote:
> > The test for database-wide vacuum is:
> >
> > database age > big_constant
> >
> > where:
> >
> > big_constant = (MaxTransactionId >> 3) * 3 - 100000) = 1610512733
>
> Ok, I'll bite... any idea how (MaxTransactionId >> 3) * 3 was chosen? I
> don't see anything about it in vacuum.c...

I don't remember exactly why was that constant chosen. IIRC Tom pulled
it out of thin air. You can see in vac_truncate_clog (vacuum.c) a
similar constant is used to emit messages about impending wraparound;
it's ((MaxTransactionId >> 3) * 3). The one in autovacuum.c allows for
more slack. (Which is kinda right because we want autovac to take care
of it _before_ the user sees any warning.)

> Also, maybe that should be defined in one place rather than duplicating
> it in autovacuum.c?

Hmm ... do you care enough to send a patch?

--
Alvaro Herrera http://www.CommandPrompt.com/
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