Re: Re[2]: [HACKERS] Fwd: Joins and links

> Leon <leon@udmnet.ru> writes:
> > I'm afraid that's mainly because fields in Postgres have variable
> > length and after update they go to the end of the table. Am I right?
> > In that case there could be done such referencing only with
> > tables with wixed width rows, whose updates can naturally be done
> > without moving. It is a little sacrifice, but it is worth it.
> 
> No, you are not right.  Tuple updates can *never* be done without
> moving the tuple, because the old tuple value must not be overwritten
> until and unless the transaction is committed.  (Under MVCC, it may
> need to stick around even longer than that, I believe.)  Thus, a tuple
> update would require an update (and move) of every referencing tuple,
> which could cascade into updates of tuples that reference those tuples,
> etc.

Yes, Thanks, Tom.  This is exactly the issue.


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