Re: optimizing vacuum truncation scans

On Wed, Jul 22, 2015 at 6:59 AM, Robert Haas <robertmhaas@gmail.com> wrote:

On Mon, Jun 29, 2015 at 1:54 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
> Attached is a patch that implements the vm scan for truncation.  It
> introduces a variable to hold the last blkno which was skipped during the
> forward portion.  Any blocks after both this blkno and after the last
> inspected nonempty page (which the code is already tracking) must have been
> observed to be empty by the current vacuum.  Any other process rendering the
> page nonempty are required to clear the vm bit, and no other process can set
> the bit again during the vacuum's lifetime.  So if the bit is still set, the
> page is still empty without needing to inspect it.

Urgh.  So if we do this, that forever precludes having HOT pruning set
the all-visible bit. 

I wouldn't say forever, as it would be easy to revert the change if something more important came along that conflicted with it.