Re: [HACKERS] Concurrency control questions 6.3.2 vs. 6.4

> 
> Hello:
> 
> I noticed the locking code in the backend/storage/lmgr directory has had a
> lot of modifications between 6.3.2 vs. 6.4.  I know that Vadim is working
> on changing the table-level locking scheme of 6.3.2 towards a
> multi-version concurrency control scheme.  I'm wondering how much along
> these modifications are -- it looks like there were changes made to the
> existing locking scheme but no additional features were added.  This is
> based on a very cursory look at the locking code in 6.4 (the locking code
> is a lot more complicated than I had initially thought it was going to
> be).
> 
> I'm curious as to how the multi-version scheme will be implemented.  Vadim
> said that Postgres has a non-overwriting storage manager which can be
> exploited for this concurrency control scheme.  I'm not sure I understand
> him -- values that are updated in a table are written to the database in
> such a fashion that the old value remains accessible?  This is
> accomplished without a recovery log?
> 
> Also, there is some user-level locking code in the contrib directory by
> Massimo that (if I am correct in my understanding of it), seems to be
> providing row-level locking capabilities through query selects.  Is this
> something that will be added to the Postgresql core at a future date?

No, this isn't row-level locking, it is a non-blocking mechanism which
can be used by applications to signal that some entities should not be
modified by other users because they are user-locked by one application
instance.
This is totally transparent and orthogonal with respect to standard locks.

-- 
Massimo Dal Zotto

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