| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | pgsql-hackers(at)postgresql(dot)org |
| Cc: | Florian Pflug <fgp(at)phlo(dot)org>, Robert Haas <robertmhaas(at)gmail(dot)com> |
| Subject: | Re: Catalog/Metadata consistency during changeset extraction from wal |
| Date: | 2012-06-21 15:40:08 |
| Message-ID: | 201206211740.09128.andres@2ndquadrant.com |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
On Thursday, June 21, 2012 03:56:54 PM Florian Pflug wrote:
> On Jun21, 2012, at 13:41 , Andres Freund wrote:
> > 3b)
> > Ensure that enough information in the catalog remains by fudging the xmin
> > horizon. Then reassemble an appropriate snapshot to read the catalog as
> > the tuple in question has seen it.
>
> The ComboCID machinery makes that quite a bit harder, I fear. If a tuple is
> updated multiple times by the same transaction, you cannot decide whether a
> tuple was visible in a certain snapshot unless you have access to the
> updating backend's ComboCID hash.
Thats a very good point. Not sure how I forgot that.
It think it might be possible to reconstruct a sensible combocid mapping from
the walstream. Let me think about it for a while...
Andres
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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