Re: 64-bit XIDs again

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.

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.

- Heikki