On Wed, Jul 8, 2015 at 10:10 PM, Sawada Masahiko <sawada.m...@gmail.com> wrote:
> On Thu, Jul 9, 2015 at 4:31 AM, Jeff Janes <jeff.ja...@gmail.com> wrote: > > On Fri, Jul 3, 2015 at 1:25 AM, Sawada Masahiko <sawada.m...@gmail.com> > > wrote: > >> > >> It's impossible to have VM bits set to frozen but not visible. > >> These bit are controlled independently. But eventually, when > >> all-frozen bit is set, all-visible is also set. > > > > > > If that combination is currently impossible, could it be used indicate > that > > the page is all empty? > > Yeah, the status of that VM bits set to frozen but not visible is > impossible, so we could use this status for another something status > of the page. > > > Having a crash-proof bitmap of all-empty pages would make vacuum > truncation > > scans much more efficient. > > The empty page is always marked all-visible by vacuum today, it's not > enough? > The "current" vacuum can just remember that they were empty as well as all-visible. But the next vacuum that occurs on the table won't know that they are empty, just that they are all-visible, so it can't truncate them away without having to read each one first. It is a minor thing, but if there is no other use for this fourth "bit-space", it seems a shame to waste it when there is some use for it. I haven't looked at the code around this area to know how hard it would be to implement the setting and clearing of the bit. Cheers, Jeff
