Re: 64-bit XIDs again

On Thu, Jul 30, 2015 at 5:24 PM, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

> On 07/30/2015 04:26 PM, Alexander Korotkov wrote:
>
>> Also, I think it's possible to migrate to 64-bit XIDs without breaking
>> pg_upgrade. Old tuples can be leaved with 32-bit XIDs while new tuples
>> would be created with 64-bit XIDs. We can use free bits in t_infomask2 to
>> distinguish old and new formats.
>>
>
> 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.
>

Nice idea. Storing extra epoch would be extra 4 bytes per heap tuple
instead of extra 8 bytes per tuple if storing 64 bits xmin/xmax.
But if first column is aligned to 8 bytes (i.e. bigserial) would we loose
this 4 bytes win for alignment?

There is a big downside to expanding xmin/xmax to 64 bits: it takes space.
> More space means more memory needed for caching, more memory bandwidth,
> more I/O, etc.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company