Re: LOCK for non-tables

At present, XC does not seem to need locks to maintain cluster-wide
integrity because it is maintained centrally in GTM. If application
needs to do this, for example, to synchronize between different
clusters, XC needs this capability obviously.
----------
Koichi Suzuki

2011/1/11 Tatsuo Ishii <ishii(at)postgresql(dot)org>:
>> On Fri, Jan 7, 2011 at 6:28 PM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
>>> I forgot about sequences earlier. If we dump while someone deletes all
>>> rows and resets the sequence the dump might contain rows and still
>>> reset the sequence. This could cause duplicate key errors on restore.
>>> I haven't checked if this is really possible though - I guess it would
>>> depend on the exact order of these events...
>>
>> To fix this, you'd need a lock that allowed getting values from the
>> sequence but prevented resetting it, and...
>>
>>> I wonder how such locks would work. Would such locks prevent accessing
>>> these objects? Or just modifications? For example, if I locked a function,
>>> could someone else execute it while I held the lock?
>>
>> ...in fact we do very little locking of objects other than tables.
>> DROP takes an AccessExclusiveLock on whatever it's dropping, and
>> COMMENT and SECURITY LABEL take ShareUpdateExclusiveLocks to avoid
>> orphaning pg_{sh,}description or pg_seclabel entries in the face of a
>> concurrent drop, but most operations on non-table objects don't AFAIK
>> take any lock at all.  We probably don't want to make too many changes
>> in this area, because there are already workloads where the
>> heavyweight lock manager can become a bottleneck, and one can easily
>> imagine that locking operators or namespaces could make that problem
>> much worse.
>
> For query based replication tools like pgpool-II (I don't know any
> other tools, for example Postgres XC falls in this category or
> not...), we need to be able to lock sequences. Fortunately it is allowed to:
>
> SELECT 1 FROM foo_sequece FOR UPDATE;
>
> but LOCK foo_sequence looks more appropreate syntax for me.
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese: http://www.sraoss.co.jp
>
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