Re: 64-bit XIDs again

On 07/30/2015 07:24 AM, Heikki Linnakangas wrote:
> 
> I think we should move to 64-bit XIDs in in-memory structs snapshots,
> proc array etc. And expand clog to handle 64-bit XIDs. But keep the
> xmin/xmax fields on heap pages at 32-bits, and add an epoch-like field
> to the page header so that logically the xmin/xmax fields on the page
> are 64 bits wide, but physically stored in 32 bits. That's possible as
> long as no two XIDs on the same page are more than 2^31 XIDs apart. So
> you still need to freeze old tuples on the page when that's about to
> happen, but it would make it possible to have more than 2^32 XID
> transactions in the clog. You'd never be forced to do anti-wraparound
> vacuums, you could just let the clog grow arbitrarily large

When I introduced the same idea a few years back, having the clog get
arbitrarily large was cited as a major issue.  I was under the
impression that clog size had some major performance impacts.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com