| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Simon Riggs <simon(at)2ndquadrant(dot)com>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: logical decoding - GetOldestXmin |
| Date: | 2012-12-19 00:59:10 |
| Message-ID: | 20121219005910.GC7666@awork2.anarazel.de |
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| Thread: | |
| Lists: | pgsql-hackers |
On 2012-12-18 19:56:18 -0500, Robert Haas wrote:
> On Tue, Dec 18, 2012 at 5:25 PM, anarazel(at)anarazel(dot)de
> <andres(at)anarazel(dot)de> wrote:
> > The problem is that at the time GetSnapshotData returns the xmin horizon might have gone upwards and tuples required for decoding might get removed by other backends. That needs to be prevented while holding the procarray lock exclusively.
>
> Well, for the ordinary use of GetSnapshotData(), that doesn't matter,
> because GetSnapshotData() also updates proc->xmin. If you're trying
> to store a different value in that field then of course it matters.
Absolutely right. I don't want to say there's anything wrong with it
right now. The "problem" for me is that it sets proc->xmin to the newest
value it can while I want/need the oldest valid value...
I will go with adding a already_locked parameter to GetOldestXmin then.
Thanks for the input,
Andres
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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