Horton

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

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Craig Ringer <craig@2ndquadrant.com>
Cc: Petr Jelinek <petr@2ndquadrant.com>, Simon Riggs <simon@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>, Jim Nasby <Jim.Nasby@bluetreble.com>, Álvaro Herrera <alvaro.herrera@2ndquadrant.com>
Date: 2017-03-09T18:55:25Z
Lists: pgsql-hackers 
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

Commits

  1. Add a txid_status function.

  2. Track the oldest XID that can be safely looked up in CLOG.