On Sat, Dec 14, 2013 at 06:21:18PM +0400, Alexander Korotkov wrote: > > Is that actually all that beneficial when sorting with a bog standard > > qsort() since that doesn't generally benefit from data being pre-sorted? > > I think we might need to switch to a different algorithm to really > > benefit from mostly pre-sorted input. > > > > In this patch I don't do full sort of dataset. For instance, index returns > data ordered by first column and we need to order them also by second > column. Then this node sorts groups (assumed to be small) where values of > the first column are same by value of second column. And with limit clause > only required number of such groups will be processed. But, I don't think > we should expect pre-sorted values of second column inside a group.
Nice. I imagine this would be mostly beneficial for fast-start plans, since you no longer need to sort the whole table prior to returning the first tuple. Reduced memory usage might be a factor, especially for large sorts where you otherwise might need to spool to disk. You can now use an index on (a) to improve sorting for (a,b). Cost of sorting n groups of size l goes from O(nl log nl) to just O(nl log l), useful for large n. Minor comments: I find cmpTuple a bad name. That's what it's doing but perhaps cmpSkipColumns would be clearer. I think it's worthwhile adding a seperate path for the skipCols = 0 case, to avoid extra copies. Have a nice day, -- Martijn van Oosterhout <klep...@svana.org> http://svana.org/kleptog/ > He who writes carelessly confesses thereby at the very outset that he does > not attach much importance to his own thoughts. -- Arthur Schopenhauer
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