Re: logical changeset generation v6.1

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...

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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