Have nodeSort.c use datum sorts single-value byref types

We originally did this in 91e9e89dc, but a memory leak was discovered
as I neglected to pfree the datum which is freshly allocated in
tuplesort_getdatum. Because that was discovered late in the PG15
cycle, we opted to just disable the datum sort optimisation for byref
types in 3a5817695.

As was mentioned in [1], it looks like we could really use a version
of tuplesort_getdatum which does not palloc a new Datum. nodeSort.c,
when calling tuplesort_gettupleslot passes copy==false, so it would
make sense if the datum sort variation didn't do any copying either.

In the attached patch, I've added a function named
tuplesort_getdatum_nocopy() which is the same as tuplesort_getdatum()
only without the datumCopy(). I opted for the new function rather than
a new parameter in the existing function just to reduce branching and
additional needless overhead.

I also looked at the tuplesort_getdatum() call inside
process_ordered_aggregate_single() and made a few changes there so we
don't needlessly perform a datumCopy() when we skip a Datum due to
finding it the same as the previous Datum in a DISTINCT aggregate
situation.

I was also looking at mode_final(). Perhaps that could do with the
same treatment, I just didn't touch it in the attached patch.

A quick performance test with:

create table t1 (a varchar(32) not null, b varchar(32) not null);
insert into t1 select md5((x%10)::text),md5((x%10)::text) from
generate_Series(1,1000000)x;
vacuum freeze t1;
create index on t1(a);

Yields a small speedup for the DISTINCT aggregate case.

work_mem = 256MB
query = select max(distinct a), max(distinct b) from t1;

Master:
latency average = 313.197 ms

Patched:
latency average = 304.335 ms (about 3% faster)

The Datum sort in nodeSort.c is more impressive.

query = select b from t1 order by b offset 1000000;

Master:
latency average = 344.763 ms

Patched:
latency average = 268.374 ms (about 28% faster)

I'll add this to the November CF

David

[1] https://www.postgresql.org/message-id/CAApHDvqS6wC5U==k9Hd26E4EQXH3QR67-T4=Q1rQ36NGvjfVSg@mail.gmail.com