On 2013-10-02 11:06:59 -0400, Robert Haas wrote:
> On Wed, Oct 2, 2013 at 11:05 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> > On 2013-10-02 10:56:38 -0400, Robert Haas wrote:
> >> On Tue, Oct 1, 2013 at 1:56 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> >> > On 2013-10-01 10:07:19 -0400, Robert Haas wrote:
> >> >> - It seems that HeapSatisfiesHOTandKeyUpdate is now
> >> >> HeapSatisfiesHOTandKeyandCandidateKeyUpdate. Considering I think this
> >> >> was merely HeapSatisfiesHOTUpdate a year ago, it's hard not to be
> >> >> afraid that something unscalable is happening to this function. On a
> >> >> related node, any overhead added here costs broadly; I'm not sure if
> >> >> there's enough to worry about.
> >> >
> >> > Ok, I had to think a bit, but now I remember why I think these changes
> >> > are not really problem: Neither the addition of keys nor candidate keys
> >> > will add any additional comparisons since the columns compared for
> >> > candidate keys are a subset of the set of key columns which in turn are a
> >> > subset of the columns checked for HOT. Right?
> >>
> >> TBH, my primary concern was with maintainability more than performance.
> >>
> >> On performance, I think any time you add code it's going to cost
> >> somehow. However, it might not be enough to care about.
> >
> > The easy alternative seems to be to call such a function multiple times
> > - which I think is prohibitive from a performance POV. More radically we
> > could simply compute the overall set/bitmap of differening columns and
> > then use bms_is_subset() to determine whether any index columns/key/ckey
> > columns changed. But that will do comparisons we don't do today...
>
> Yeah, there may be no better alternative to doing things as you've
> done them here. It just looks grotty, so I was hoping we had a better
> idea. Maybe not.
Imo the code now looks easier to understand - which is not saying much -
than in 9.3/HEAD...
Greetings,
Andres Freund
-- Andres Freund http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services
