Fix performance regression in tuplesort specializations

697492434 added 3 new qsort specialization functions aimed to improve the
performance of sorting many of the common pass-by-value data types when
they're the leading or only sort key.

Unfortunately, that has caused a performance regression when sorting
datasets where many of the values being compared were equal.  What was
happening here was that we were falling back to the standard sort
comparison function to handle tiebreaks.  When the two given Datums
compared equally we would incur both the overhead of an indirect function
call to the standard comparer to perform the tiebreak and also the
standard comparer function would go and compare the leading key needlessly
all over again.

Here improve the situation in the 3 new comparison functions.  We now
return 0 directly when the two Datums compare equally and we're performing
a 1-key sort.

Here we don't do anything to help the multi-key sort case where the
leading key uses one of the sort specializations functions.  On testing
this case, even when the leading key's values are all equal, there
appeared to be no performance regression.  Let's leave it up to future
work to optimize that case so that the tiebreak function no longer
re-compares the leading key over again.

Another possible fix for this would have been to add 3 additional sort
specialization functions to handle single-key sorts for these
pass-by-value types.  The reason we didn't do that here is that we may
deem some other sort specialization to be more useful than single-key
sorts.  It may be impractical to have sort specialization functions for
every single combination of what may be useful and it was already decided
that further analysis into which ones are the most useful would be delayed
until the v16 cycle.  Let's not let this regression force our hand into
trying to make that decision for v15.

Author: David Rowley
Reviewed-by: John Naylor
Discussion: https://postgr.es/m/CA+hUKGJRbzaAOUtBUcjF5hLtaSHnJUqXmtiaLEoi53zeWSizeA@mail.gmail.com

src/backend/utils/sort/tuplesort.c

@@ -436,7 +436,11 @@ struct Tuplesortstate
 
 	/*
 	 * This variable is shared by the single-key MinimalTuple case and the
-	 * Datum case (which both use qsort_ssup()).  Otherwise it's NULL.
+	 * Datum case (which both use qsort_ssup()).  Otherwise, it's NULL.  The
+	 * presence of a value in this field is also checked by various sort
+	 * specialization functions as an optimization when comparing the leading
+	 * key in a tiebreak situation to determine if there are any subsequent
+	 * keys to sort on.
 	 */
 	SortSupport onlyKey;
 
@@ -701,6 +705,13 @@ qsort_tuple_unsigned_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 	if (compare != 0)
 		return compare;
 
+	/*
+	 * No need to waste effort calling the tiebreak function when there are
+	 * no other keys to sort on.
+	 */
+	if (state->onlyKey != NULL)
+		return 0;
+
 	return state->comparetup(a, b, state);
 }
 
@@ -713,9 +724,17 @@ qsort_tuple_signed_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 	compare = ApplySignedSortComparator(a->datum1, a->isnull1,
 										b->datum1, b->isnull1,
 										&state->sortKeys[0]);
+
 	if (compare != 0)
 		return compare;
 
+	/*
+	 * No need to waste effort calling the tiebreak function when there are
+	 * no other keys to sort on.
+	 */
+	if (state->onlyKey != NULL)
+		return 0;
+
 	return state->comparetup(a, b, state);
 }
 
@@ -728,9 +747,17 @@ qsort_tuple_int32_compare(SortTuple *a, SortTuple *b, Tuplesortstate *state)
 	compare = ApplyInt32SortComparator(a->datum1, a->isnull1,
 										b->datum1, b->isnull1,
 										&state->sortKeys[0]);
+
 	if (compare != 0)
 		return compare;
 
+	/*
+	 * No need to waste effort calling the tiebreak function when there are
+	 * no other keys to sort on.
+	 */
+	if (state->onlyKey != NULL)
+		return 0;
+
 	return state->comparetup(a, b, state);
 }