Re: Maximize page freezing

On Thu, 28 Jul 2022 at 15:36, Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com> wrote:
>
> Starting new thread with updated patch to avoid confusion, as
> mentioned by David Steele on the original thread:
> Original messageid: 20201118020418(dot)GA13408(at)alvherre(dot)pgsql
> On Wed, 18 Nov 2020 at 02:04, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> > On 2020-Nov-17, Simon Riggs wrote:
> >
> > > As an additional optimization, if we do find a row that needs freezing
> > > on a data block, we should simply freeze *all* row versions on the
> > > page, not just the ones below the selected cutoff. This is justified
> > > since writing the block is the biggest cost and it doesn't make much
> > > sense to leave a few rows unfrozen on a block that we are dirtying.
> >
> > Yeah. We've had earlier proposals to use high and low watermarks: if any
> > tuple is past the high watermark, then freeze all tuples that are past
> > the low watermark. However this is ancient thinking (prior to
> > HEAP_XMIN_FROZEN) and we don't need the low watermark to be different
> > from zero, since the original xid is retained anyway.
> >
> > So +1 for this idea.
>
> Updated patch attached.

Great idea, yet this patch seems to only freeze those tuples that are
located after the first to-be-frozen tuple. It should probably
re-visit earlier live tuples to potentially freeze those as well.

Kind regards,

Matthias van de Meent