Simon Riggs wrote:
> On 28 December 2012 10:21, Simon Riggs <simon@2ndquadrant.com> wrote:
>
> > * There is a fixed 75% heuristic in the patch.
>
> I'm concerned that we're doing extra work while holding the buffer
> locked, which will exacerbate any block contention that exists.
>
> We have a list of the columns that the UPDATE is touching since we use
> that to check column permissions for the UPDATE. Which means we should
> be able to use that list to check only the columns actually changing
> in this UPDATE statement.
But that doesn't include columns changed by triggers, AFAIR, so you
could only use that if there weren't any triggers.
I was also worried about the high variance in the results. Those
averages look rather meaningless. Which would be okay, I think, because
it'd mean that performance-wise the patch is a wash, but it is still
achieving a lower WAL volume, which is good.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
