FAQ

On 30 May 2013 14:33, Heikki Linnakangas 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

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postedMay 30, '13 at 1:34p
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