Re: B-Tree support function number 3 (strxfrm() optimization)

On Tue, Sep 2, 2014 at 12:22 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Maybe we should get rid of the tiebreak case altogether: the second
> SortSupport object is just containing all the same values as the first
> one, with only the comparator being different. Can't we just have
> both the abbreviated-comparator and authoritative-comparator as
> members of the SortSupport, and call whichever one is appropriate,
> instead of copying the whole SortSupport object? That would have the
> nice property of avoiding the need for special handling in
> reversedirection_heap().

I thought about that. I think that there are other disadvantages to
explicitly having a second comparator, associated with a the same sort
support state as the authoritative comparator: ApplySortComparator()
expects to compare using ssup->comparator(). You'd have to duplicate
that for your alternative/abbreviated comparator. It might be to our
advantage to use the same ApplySortComparator() inline comparator
muliple times in routines like comparetup_heap(), if not for clarity
then for performance (granted, that isn't something I have any
evidence for, but I wouldn't be surprised if it was noticeable). It
might also be to our advantage to have a separate work space.

By having a second comparator, you're making the leading
key/abbreviated comparison special, rather than the (hopefully) less
common tie-breaker case. I find it more logical to structure the code
such that the leading/abbreviated comparison is the "regular state",
with a tie-breaker on the "irregular"/authoritative state accessed
through indirection from the leading key state, reflecting our
preference for having most comparisons resolved using cheap
abbreviated comparisons. It's not as if I feel that strongly about it,
though.

--
Peter Geoghegan