| From: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> |
|---|---|
| To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
| Cc: | Albe Laurenz <laurenz(dot)albe(at)wien(dot)gv(dot)at>, Tom Lane *EXTERN* <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Hannu Krosing <hannu(at)2ndQuadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Feedback on getting rid of VACUUM FULL |
| Date: | 2009-09-18 08:01:35 |
| Message-ID: | 4AB33E5F.20803@enterprisedb.com |
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| Lists: | pgsql-hackers |
Simon Riggs wrote:
> CTIDs don't help with optimistic locking, though it seems they can.
>
> If you don't hold open the transaction then someone else can update the
> row. That sounds good, but because of HOT it is possible that the same
> CTID with the same PK value occupies that exact CTID value when you
> return to check it. You think row has not been updated so you perform
> your update, but it has been updated, so you overwrite previous data -
> data loss. Actually worse, sporadic data loss because of race
> conditions.
Yeah, you have to check xmin as well.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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