Re: Meaning of vacuum output

Tom Lane wrote:
>
> "Jeffrey W. Baker" <jwbaker@acm.org> writes:
> > I shut down all connections, and vacuum has a different output:
>
> > NOTICE:  Pages 33639: Changed 226, Empty 0; Tup 128819: Vac 2840812,
> > Keep 0, UnUsed 96275.
> >         Total CPU 14.29s/28.69u sec elapsed 233.02 sec.
>
> Ah-hah, you did have open transactions.
>
> > So I conclude from these two data points that Keep means tuples that are
> > still visible to a transaction, Vac means tuples that are not visible to
> > any transaction.  But, what is UnUsed?
>
> Looking at the code:
>
> Tup             # tuples remaining after vacuum
> Vac             # tuples removed by vacuum
> Keep            # dead tuples kept because some xact can still see 'em
> Unused          # unused item pointers
>
> "Keep" is included in the "Tup" total.
>
> The unused item pointers may get recycled later; it looks like that
> hasn't been determined yet at the point where these stats are printed.

I think this explanation would be worth mentioning in the documentation
for the VACUUM command (maybe as OUTPUT?).

???

Regards and best wishes,

Justin Clift

>
>                         regards, tom lane
>
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