Improve performance of our private version of qsort. Per recent testing,

the logic it contained to switch to insertion sort for near-sorted input was
in fact a big loss, because it could fairly easily be fooled into applying
insertion sort to large subfiles that weren't all that well ordered.  Remove
that, and instead add a simple check for already-perfectly-sorted input, as
per suggestion from Dann Corbit.  This adds at worst O(N*lgN) overhead, and
usually far less, while sometimes allowing a subfile sort to finish in O(N)
time.  Preliminary testing says this is an improvement over the basic
Bentley & McIlroy code for many nonrandom inputs, and it costs almost
nothing when the input is random.

src/port/qsort.c

@@ -2,8 +2,10 @@
  *	Modifications from vanilla NetBSD source:
  *	  Add do ... while() macro fix
  *	  Remove __inline, _DIAGASSERTs, __P
+ *	  Remove ill-considered "swap_cnt" switch to insertion sort,
+ *	  in favor of a simple check for presorted input.
  *
- *	$PostgreSQL: pgsql/src/port/qsort.c,v 1.8 2005/10/15 02:49:51 momjian Exp $
+ *	$PostgreSQL: pgsql/src/port/qsort.c,v 1.9 2006/03/21 19:49:15 tgl Exp $
  */
 
 /*	$NetBSD: qsort.c,v 1.13 2003/08/07 16:43:42 agc Exp $	*/
@@ -47,7 +49,11 @@ static void swapfunc(char *, char *, size_t, int);
 #define min(a, b)	((a) < (b) ? (a) : (b))
 
 /*
- * Qsort routine from Bentley & McIlroy's "Engineering a Sort Function".
+ * Qsort routine based on J. L. Bentley and M. D. McIlroy,
+ * "Engineering a sort function",
+ * Software--Practice and Experience 23 (1993) 1249-1265.
+ * We have modified their original by adding a check for already-sorted input,
+ * which seems to be a win per discussions on pgsql-hackers around 2006-03-21.
  */
 #define swapcode(TYPE, parmi, parmj, n) \
 do {		\
@@ -116,10 +122,9 @@ int			(*cmp) (const void *, const void *);
 	int			d,
 				r,
 				swaptype,
-				swap_cnt;
+				presorted;
 
 loop:SWAPINIT(a, es);
-	swap_cnt = 0;
 	if (n < 7)
 	{
 		for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
@@ -128,6 +133,17 @@ loop:SWAPINIT(a, es);
 				swap(pl, pl - es);
 		return;
 	}
+	presorted = 1;
+	for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
+	{
+		if (cmp(pm - es, pm) > 0)
+		{
+			presorted = 0;
+			break;
+		}
+	}
+	if (presorted)
+		return;
 	pm = (char *) a + (n / 2) * es;
 	if (n > 7)
 	{
@@ -144,7 +160,6 @@ loop:SWAPINIT(a, es);
 	}
 	swap(a, pm);
 	pa = pb = (char *) a + es;
-
 	pc = pd = (char *) a + (n - 1) * es;
 	for (;;)
 	{
@@ -152,7 +167,6 @@ loop:SWAPINIT(a, es);
 		{
 			if (r == 0)
 			{
-				swap_cnt = 1;
 				swap(pa, pb);
 				pa += es;
 			}
@@ -162,7 +176,6 @@ loop:SWAPINIT(a, es);
 		{
 			if (r == 0)
 			{
-				swap_cnt = 1;
 				swap(pc, pd);
 				pd -= es;
 			}
@@ -171,19 +184,9 @@ loop:SWAPINIT(a, es);
 		if (pb > pc)
 			break;
 		swap(pb, pc);
-		swap_cnt = 1;
 		pb += es;
 		pc -= es;
 	}
-	if (swap_cnt == 0)
-	{							/* Switch to insertion sort */
-		for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
-			for (pl = pm; pl > (char *) a && cmp(pl - es, pl) > 0;
-				 pl -= es)
-				swap(pl, pl - es);
-		return;
-	}
-
 	pn = (char *) a + n * es;
 	r = min(pa - (char *) a, pb - pa);
 	vecswap(a, pb - r, r);