On Sat, Aug 10, 2019 at 1:20 AM Heikki Linnakangas <hlinn...@iki.fi> wrote: > Hmm. Wouldn't it be more straightforward to have the extra tupindex > field at the end of the struct?
> The initial sorting phase would deal with SortTuples, and the merge > phase would deal with MergeTuples. The same comparison routines work > with both. Maybe, but then you would have to use MergeTuples in the tuplesort_heap* routines, which are not just used when merging external sort runs. You'd probably incur a penalty for top-N heap sorts too. Now, that could still be worth it, but it's something to consider. > If you separate the NULLs from non-NULLs in a separate array, as was > discussed back in 2016, instead of stealing a bit, you can squeeze some > instructions out of the comparison routines, which might give some extra > speedup. That might work well, but partitioning the memtuples array isn't trivial. Routines like grow_memtuples() still need to work, and that seems like it would be tricky. So again, this may well be a better way to do it, but that isn't obvious. > > But in cases that users really care about, such as REINDEX, > > the difference is in the noise. ISTM that this is simple not worth the > > trouble at this time. These days, external sorts are often slightly > > faster than internal sorts in practice, due to the fact that we can do > > an on-the-fly merge with external sorts, so we could easily hurt > > performance by making more memory available! > > Yeah, that's a bit sad. I think that this is likely to be the problem with any combination of enhancements that remove fields from the SortTuple struct, to get it down to 16 bytes: Making SortTuples only 16 bytes just isn't that compelling. > That makes me think: even when everything fits in memory, it might make > sense to divide the input into a few batches, qsort them individually, > and do an on-the-fly merge of the batches. I guess I'm essentially > suggesting that we should use merge instead of quicksort for the > in-memory case, too. That might make sense. The Alphasort paper [1] recommends using quicksort on CPU-cached sized chunks, and merging the chunks together as they're written out as a single on-disk run. The Alphasort paper is probably the first place where the abbreviated keys technique is described, and had a lot of good ideas. > If we had the concept of in-memory batches, you could merge together > in-memory and external batches. That might be handy. For example, when > doing an external sort, instead of flushing the last run to disk before > you start merging, you could keep it in memory. That might be > significant in the cases where the input is only slightly too big to fit > in memory. The patch that I wrote to make tuplesort.c use quicksort in preference to replacement selection sort for generating initial runs starting out with an implementation of something that I called "quicksort with spillover". The idea was that you could only spill a few extra tuples to disk when you almost had enough workMem, and then merge the on-disk run with the larger, quicksorted in memory run. It worked alright, but it felt more important to make external sorts use quicksort in general. Robert Haas really hated it at the time, because it relied on various magic numbers, based on heuristics. The easiest and least controversial way to make internal sorting faster may be to update our Quicksort algorithm to use the same implementation that was added to Java 7 [2]. It uses all of the same tricks as our existing the Bentley & McIlroy implementation, but is more cache efficient. It's considered the successor to B&M, and had input from Bentley himself. It is provably faster than B&M for a wide variety of inputs, at least on modern hardware. [1] http://www.vldb.org/journal/VLDBJ4/P603.pdf [2] https://codeblab.com/wp-content/uploads/2009/09/DualPivotQuicksort.pdf -- Peter Geoghegan