Re: Challenges preventing us moving to 64 bit transaction id (XID)?

On 6 June 2017 at 12:38, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> On Tue, Jun 6, 2017 at 10:00 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> writes:
>>> On Tue, Jun 6, 2017 at 9:48 AM, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>>>> Storing an epoch implies that rows can't have (xmin,xmax) different by
>>>> more than one epoch. So if you're updating/deleting an extremely old
>>>> tuple you'll presumably have to set xmin to FrozenTransactionId if it
>>>> isn't already, so you can set a new epoch and xmax.
>>
>>> If the page has multiple such tuples, updating one tuple will mean
>>> updating headers of other tuples as well? This means that those tuples
>>> need to be locked for concurrent scans?
>>
>> Locks for tuple header updates are taken at page level anyway, so in
>> principle you could run around and freeze other tuples on the page
>> anytime you had to change the page's high-order-XID value. Holding
>> the lock for long enough to do that is slightly annoying, but it
>> should happen so seldom as to not represent a real performance problem.
>>
>> In my mind the harder problem is where to find another 32 bits for the
>> new page header field. You could convert the header format on-the-fly
>> if there's free space in the page, but what if there isn't?
>
> I guess, we will have to reserve 32 bits in the header. That's much
> better than increasing tuple header by 32 bits.

Tom's point is, I think, that we'll want to stay pg_upgrade
compatible. So when we see a pg10 tuple and want to add a new page
with a new page header that has an epoch, but the whole page is full
so there isn't 32 bits left to move tuples "down" the page, what do we
do?

--
Craig Ringer http://www.2ndQuadrant.com/
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