Re: [PATCH] Use optimized single-datum tuplesort in ExecSort

On Sat, 17 Jul 2021 at 01:14, James Coleman <jtc331(at)gmail(dot)com> wrote:
> The only remaining question I have is whether or not costing needs to
> change, given the very significant speedup for datum sort.

I'm looking at cost_tuplesort and the only thing that I think might
make sense would be to adjust how the input_bytes value is calculated.
For now, that's done with the following function that's used in quite
a number of places.

static double
relation_byte_size(double tuples, int width)
{
return tuples * (MAXALIGN(width) + MAXALIGN(SizeofHeapTupleHeader));
}

It seems, at least in the case of Sort, that using SizeofHeapTupleHead
is just always wrong as it should be SizeofMinimalTupleHeader. I know
that's also the case for Memoize too. I've not checked the other
locations.

The only thing I can really see that we might do would be not add the
MAXALIGN(SizeofHeapTupleHeader) when there's just a single column.
We'd need to pass down the number of attributes from
create_sort_path() so we'd know when and when not to add that. I'm not
saying that we should do this. I'm just saying that I don't really see
what else we might do.

I can imagine another patch might just want to do a complete overhaul
of all locations that use relation_byte_size(). There are various
things that function just does not account for. e.g, the fact that we
allocate chunks in powers of 2 and that there's a chunk header added
on. Of course, "width" is just an estimate, so maybe trying to
calculate something too precisely wouldn't be too wise. However,
there's a bit of a chicken and the egg problem there as there'd be
little incentive to improve "width" unless we started making more
accurate use of the value.

Anyway, none of the above take into account that the Datum sort is
just a little faster, The only thing that exists in the existing cost
modal that we could use to adjust the cost of an in memory sort is the
comparison_cost. The problem there is that the comparison is exactly
the same in both Datum and Tuple sorts. The only thing that really
changes between Datum and Tuple sort is the fact that we don't make a
MinimalTuple when doing a Datum sort. The cost modal, unfortunately,
does not account for that. That kinda makes me think that we should
do nothing as if we start to account for making MemoryTuples then
we'll just penalise Tuple sorts and that might cause someone to be
upset.

David