Re: Is the "ACCESS EXCLUSIVE" lock for TRUNCATE really

Tom Lane wrote:
> "Florian G. Pflug" <fgp@phlo.org> writes:
>>Tom Lane wrote:
>>
>>>Until when?  How would you synchronize the switchover?
>
>>Every snapshot would either contain the old, or the new version of
>>the corresponding pg_class tuple. The ones using the old version
>>couldn't possible be writer, only reader (TRUNCATE would still need
>>to acquire a lock that ensures that). New transactions started after
>>the commit of the truncate would see the new version, and use
>>the new datafile.
>
> Wrong.  *All* transactions read the system catalogs with SnapshotNow.
Ah, well that clearly kills my idea... Too bad...

I was fooled by the fact that most ddl-statements can be rolled back,
and assumed that this follows from using "normal" mvcc semantics when
reading the catalog tables.

Thanks for your explanations!

greetings, Florian Pflug