Re: VACUUM optimization ideas.

Alfred Perlstein wrote:
> #1
> 
> Reducing the time vacuum must hold an exlusive lock on a table:
> 
> The idea is that since rows are marked deleted it's ok for the
> vacuum to fill them with data from the tail of the table as
> long as no transaction is in progress that has started before
> the row was deleted.
> 
> This may allow the vacuum process to copyback all the data without
> a lock, when all the copying is done it then aquires an exlusive lock
> and does this:
> 
> Aquire an exclusive lock.
> Walk all the deleted data marking it as current.
> Truncate the table.
> Release the lock.
> 
> Since the data is still marked invalid (right?) even if valid data
> is copied into the space it should be ignored as long as there's no
> transaction occurring that started before the data was invalidated.

Yes, but nothing prevents newer transactions from modifying the _origin_ side of
the copied data _after_ it was copied, but before the Lock-Walk-Truncate-Unlock
cycle takes place, and so it seems unsafe. Maybe locking each record before
copying it up ...

Regards,
Haroldo.

-- 
----------------------+------------------------Haroldo Stenger      | hstenger@ieee.orgMontevideo, Uruguay. |
hstenger@adinet.com.uy
----------------------+------------------------Visit UYLUG Web Site: http://www.linux.org.uy
-----------------------------------------------