On 13 April 2012 17:42, Peter Geoghegan <peter@2ndquadrant.com> wrote:
> One insight that I had at the time was that text comparisons where the
> c locale isn't used are really rather expensive, and I doubt that
> there is too much that can be done to address that directly. However,
> if we were to support timsort, that could substantially cut down on
> the number of comparisons for text columns, which could be a real win.
> Maybe that would happen through some explicit mechanism, or maybe the
> planner could actually infer that it was likely to be optimal to use
> timsort based on a high statistical correlation between physical row
> ordering and logical ordering of a key column's values.
Further thoughts:
At the time we committed our own quicksort implementation, based on
the NetBSD one, we eschewed the optimisation of using insertion sort
when n is fairly low. This happens to be a very common optimisation,
so I'm not really super-confident that that was a good decision.
However, we also added our own optimisation, which is to attempt,
regardless of the size of n, to ascertain that the array is
pre-sorted, in which case we don't quicksort at all.
So if we attempt to quicksort an array which is almost pre-sorted, but
say only has its very last element out-of-order, we'll do fairly
horribly, because we waste the effort of almost an entire "bubble sort
iteration". So almost-sorted data would seem to be a compelling thing
to optimise for that reason, as well as for the simple observation
that it isn't exactly uncommon in a relational database.
--
Peter Geoghegan http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services
