Re: [HACKERS] [PATCH] Transaction traceability - txid_status(bigint)

On Wed, Jan 25, 2017 at 12:44 AM, Craig Ringer <craig@2ndquadrant.com> wrote:
>> The way that SetTransactionIdLimit() now works looks a bit dangerous.
>> xidWrapLimit, xidStopLimit, and xidWarnLimit are computed based on the
>> passed-in oldestXid value and written straight into shared memory.
>> But the shared memory copy of oldestXid could have a different value.
>> I'm not sure if that breaks anything, but it certainly weakens any
>> confidence callers might have had that all those values are consistent
>> with each other.
>
> This was my main hesitation with the whole thing too.
>
> It's necessary to advance oldestXmin before we xlog the advance and
> truncate clog, and necessary to advance the vacuum limits only
> afterwards.

Well, that's why I tried to advocate that their should be two separate
XID limits in shared memory: leave what's there now the way it is, and
then add a new limit for "don't try to look up XIDs before this point:
splat".  I still think that'd be less risky.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company