Create a "sort support" interface API for faster sorting.

This patch creates an API whereby a btree index opclass can optionally
provide non-SQL-callable support functions for sorting.  In the initial
patch, we only use this to provide a directly-callable comparator function,
which can be invoked with a bit less overhead than the traditional
SQL-callable comparator.  While that should be of value in itself, the real
reason for doing this is to provide a datatype-extensible framework for
more aggressive optimizations, as in Peter Geoghegan's recent work.

Robert Haas and Tom Lane

src/backend/utils/adt/date.c

@@ -29,6 +29,7 @@
 #include "utils/date.h"
 #include "utils/datetime.h"
 #include "utils/nabstime.h"
+#include "utils/sortsupport.h"
 
 /*
  * gcc's -ffast-math switch breaks routines that expect exact results from
@@ -320,6 +321,28 @@ date_cmp(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32(0);
 }
 
+static int
+date_fastcmp(Datum x, Datum y, SortSupport ssup)
+{
+	DateADT		a = DatumGetDateADT(x);
+	DateADT		b = DatumGetDateADT(y);
+
+	if (a < b)
+		return -1;
+	else if (a > b)
+		return 1;
+	return 0;
+}
+
+Datum
+date_sortsupport(PG_FUNCTION_ARGS)
+{
+	SortSupport	ssup = (SortSupport) PG_GETARG_POINTER(0);
+
+	ssup->comparator = date_fastcmp;
+	PG_RETURN_VOID();
+}
+
 Datum
 date_finite(PG_FUNCTION_ARGS)
 {