Shave a few cycles in compare_pathkeys() by checking for pointer-identical

input lists before we grovel through the lists.  This doesn't save much,
but testing shows that the case of both inputs NIL is common enough that
it saves something.  And this is used enough to be a hotspot.

src/backend/optimizer/path/pathkeys.c

@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.96 2009/01/01 17:23:44 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.97 2009/02/28 03:51:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -332,6 +332,14 @@ compare_pathkeys(List *keys1, List *keys2)
 	ListCell   *key1,
 			   *key2;
 
+	/*
+	 * Fall out quickly if we are passed two identical lists.  This mostly
+	 * catches the case where both are NIL, but that's common enough to
+	 * warrant the test.
+	 */
+	if (keys1 == keys2)
+		return PATHKEYS_EQUAL;
+
 	forboth(key1, keys1, key2, keys2)
 	{
 		PathKey    *pathkey1 = (PathKey *) lfirst(key1);
@@ -354,11 +362,11 @@ compare_pathkeys(List *keys1, List *keys2)
 	 * If we reached the end of only one list, the other is longer and
 	 * therefore not a subset.
 	 */
-	if (key1 == NULL && key2 == NULL)
-		return PATHKEYS_EQUAL;
 	if (key1 != NULL)
 		return PATHKEYS_BETTER1;	/* key1 is longer */
-	return PATHKEYS_BETTER2;	/* key2 is longer */
+	if (key2 != NULL)
+		return PATHKEYS_BETTER2;	/* key2 is longer */
+	return PATHKEYS_EQUAL;
 }
 
 /*