Re: vacuum, performance, and MVCC

On 6/23/06, Mark Woodward wrote:
>
> For each update to a row additional work needs to be done to access that
> row. Surely a better strategy can be done, especially considering that the
> problem being solved is a brief one.
>
> The only reason why you need previous versions of a row is for
> transactions that started before or during the transaction that seeks to
> modify a row. After which time, the previous versions continue to affect
> performance and take up space even though they are of no value.
> (Caveats for rollback, etc. but the point is still valid).
>
> This is a very pessimistic behavior and penalizes the more common and
> optimistic operations.

Are you sure about that? ISTM that for the most common cases the TID
returned by an indexscan is the one of the last version and only if
that vbersion is too new a second TID is tried etc.

Jochem