On Thu, Feb 18, 2016 at 4:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Only a physical-order scan, ie vacuum, would visit a dead page
> (ignoring transient corner cases like a page getting deleted while an
> indexscan is in flight to it). So I think treating it as part of the
> fragmentation measure is completely wrong: the point of that measure,
> AFAICS, is to model how close an index-order traversal is to linear.
> Half-dead pages are also normally very transient --- the only way they
> persist is if there's a crash partway through a page deletion. So I think
> it's appropriate to assume that future indexscans won't visit those,
> either.
Okay.
>> there are usage patterns where half-dead pages might accumulate.
>
> Other than a usage pattern of "randomly SIGKILL backends every few
> seconds", I don't see how that would happen.
I meant where pages could accumulate without being recycled.
--
Peter Geoghegan