Ühel kenal päeval, R, 2006-11-10 kell 12:19, kirjutas Simon Riggs:
> On Thu, 2006-11-09 at 18:28 -0500, Tom Lane wrote:

> > > HOT can only work in cases where a tuple does not modify one of the
> > > columns defined in an index on the table, and when we do not alter the
> > > row length of the tuple.
> > 
> > Seems like "altering the row length" isn't the issue, it's just "is
> > there room on the page for the new version".  Again, a generous
> > fillfactor would give you more flexibility.
> The copy-back operation can only work if the tuple fits in the same
> space as the root tuple. If it doesn't you end up with a tuple
> permanently in the overflow relation. That might not worry us, I guess.
> Also, my understanding was that an overwrite operation could not vary
> the length of a tuple (at least according to code comments).

But you can change the t_ctid pointer in the heap tuple to point to
oldest visible tuple in overflow.

What I did not understand very well, is how do you determine, which
overflow tuples are safe to remove. Do you mark/remove them when doing
the copyback ?

Hannu Krosing
Database Architect
Skype Technologies OÜ
Akadeemia tee 21 F, Tallinn, 12618, Estonia

Skype me:  callto:hkrosing
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