On 30 May 2013 14:33, Heikki Linnakangas <hlinnakangas@vmware.com> wrote:
> Since we're bashing around ideas around freezing, let me write down the idea
> I've been pondering and discussing with various people for years. I don't
> think I invented this myself, apologies to whoever did for not giving
> credit.
>
> The reason we have to freeze is that otherwise our 32-bit XIDs wrap around
> and become ambiguous. The obvious solution is to extend XIDs to 64 bits, but
> that would waste a lot space. The trick is to add a field to the page header
> indicating the 'epoch' of the XID, while keeping the XIDs in tuple header
> 32-bit wide (*).
>
> The other reason we freeze is to truncate the clog. But with 64-bit XIDs, we
> wouldn't actually need to change old XIDs on disk to FrozenXid. Instead, we
> could implicitly treat anything older than relfrozenxid as frozen.
>
> That's the basic idea. Vacuum freeze only needs to remove dead tuples, but
> doesn't need to dirty pages that contain no dead tuples.
I have to say this is pretty spooky. I'd not read hackers all week, so
I had no idea so many other people were thinking about freezing as
well. This idea is damn near identical to what I've suggested. My
suggestion came because I was looking to get rid of fields out of the
tuple header; which didn't come to much. The good news is that is
complete chance, so it must mean we're on the right track.
--Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services