Re: [Postgres-xc-developers] [postgres-xc:bugs] #454 Update triggers on replicated tables may corrupt data

[Andrei M. <and...@gm...>]

I believe even plain vacuum may move tuples within a page and purge empty
pages, therefore CTIDs could be changed.
Autovacuum on Datamodes are not in sync and concurrent writes may occur in
different order on Datanodes.
CTID can not be used to identify tuple replicas. Primary key could be used,
or special system column. Otherwise not shippable updates against
replicated tables should not be allowed.


2013/10/9 Pavan Deolasee <pav...@gm...>

>
>
>
> On Wed, Oct 9, 2013 at 11:49 AM, Koichi Suzuki <koi...@gm...>wrote:
>
>> Hmm.. Yes, it could be serious.   I suppose that CTID changes only when
>> vacuum full occurs.
>>
>
> CTID will also change every time a row is updated. So the bug should be
> very easy to reproduce. One easiest way to compile different datanodes with
> diferent block size. Another way could be to update/delete from a
> replicated table and vacuum the table on one datanode but not other and
> then again update several rows in the table. Most likely the new versions
> on two different nodes will not have the same CTID anymore.
>
>
>> Vacuum freeze will updates xmim to the smallest value and usual vacuum
>> recycles "dead" row area.   We may have a chance to assign different CTIDs
>> to the same rows in replicated table if vacuum full runs locally in
>> datanodes.  (My assumption may be wrong.   Does vacuum freeze move tuples
>> and change their CTID?)
>>
>>
> No, it does not.
>
> Thanks,
> Pavan
>
> --
> Pavan Deolasee
> http://www.linkedin.com/in/pavandeolasee
>
>
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-- 
Andrei Martsinchyk

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