Re: GSOC 2018 Project - A New Sorting Routine

On Fri, Jul 13, 2018 at 3:04 PM, Kefan Yang <starordust@gmail.com> wrote:
> 1. Slow on CREATE INDEX cases.
>
> I am still trying to figure out where the bottleneck is. Is the data pattern
> in index creation very different from other cases? Also, pg_qsort has
> 10%-20% advantage at creating index even on sorted data (faster CPU, N =
> 1000000). This is very strange to me since the two sorting routines execute
> exactly the same code when the input data is sorted.

Yes. CREATE INDEX uses heap TID as a tie-breaker, so it's impossible
for any two index tuples to compare as equal within tuplesort.c, even
though they may be equal in other contexts. This is likely to defeat
things like the Bentley-McIlroy optimization where equal keys are
swapped, which is very effective in the event of many equal keys.

(Could also be parallelism, though I suppose you probably accounted for that.)

-- 
Peter Geoghegan