Re: TOAST code ignores freespace (was Tweak TOAST code)

On Sun, 2010-05-02 at 10:34 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndQuadrant.com> writes:
> > Not commenting further on that patch, but I notice that when we UPDATE
> > the toasting algorithm takes no account of the available freespace on
> > the current block. If we are updating and the space available would make
> > a difference to the row length chosen, it seems like it would be more
> > beneficial to trim the row and encourage HOT updates.
> 
> That doesn't strike me as a terribly good idea: it would make the
> behavior of TOAST significantly more difficult to predict.  Also, what
> happens if we force a row to a smaller size and then it doesn't fit
> anyway (eg because someone else inserted another row on the page while
> we were busy doing this)?  Spend even more cycles to un-toast back to
> the normal size, to be consistent with ordinary cross-page updates?
> 
> Pretty much every previous discussion of tweaking the TOAST behavior
> has focused on giving the user more control (indeed, the patch you
> mention could be seen as doing that).  What you're suggesting here
> would give the user less control, as well as less predictability.

As long as we've considered it, I'm happy either way. You know I'm
happier with more user control.

-- Simon Riggs           www.2ndQuadrant.com