Use fuzzy comparison of path costs in add_path(), so that paths with the

same path keys and nearly equivalent costs will be considered redundant.
The exact nature of the fuzziness may get adjusted later based on current
discussions, but no one has shot a hole in the basic idea yet ...

Author: tglsfdc

src/backend/optimizer/util/pathnode.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/pathnode.c,v 1.102 2004/03/02 16:42:20 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/pathnode.c,v 1.103 2004/03/29 19:58:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -84,6 +84,76 @@ compare_path_costs(Path *path1, Path *path2, CostSelector criterion)
 	return 0;
 }
 
+/*
+ * compare_fuzzy_path_costs
+ *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
+ *	  or more expensive than path2 for the specified criterion.
+ *
+ * This differs from compare_path_costs in that we consider the costs the
+ * same if they agree to within a "fuzz factor".  This is used by add_path
+ * to avoid keeping both of a pair of paths that really have insignificantly
+ * different cost.
+ */
+static int
+compare_fuzzy_path_costs(Path *path1, Path *path2, CostSelector criterion)
+{
+	Cost		fuzz;
+
+	/*
+	 * The fuzz factor is set at one percent of the smaller total_cost, but
+	 * not less than 0.01 cost units (just in case total cost is zero).
+	 *
+	 * XXX does this percentage need to be user-configurable?
+	 */
+	fuzz = Min(path1->total_cost, path2->total_cost) * 0.01;
+	fuzz = Max(fuzz, 0.01);
+
+	if (criterion == STARTUP_COST)
+	{
+		if (Abs(path1->startup_cost - path2->startup_cost) > fuzz)
+		{
+			if (path1->startup_cost < path2->startup_cost)
+				return -1;
+			else
+				return +1;
+		}
+
+		/*
+		 * If paths have the same startup cost (not at all unlikely),
+		 * order them by total cost.
+		 */
+		if (Abs(path1->total_cost - path2->total_cost) > fuzz)
+		{
+			if (path1->total_cost < path2->total_cost)
+				return -1;
+			else
+				return +1;
+		}
+	}
+	else
+	{
+		if (Abs(path1->total_cost - path2->total_cost) > fuzz)
+		{
+			if (path1->total_cost < path2->total_cost)
+				return -1;
+			else
+				return +1;
+		}
+
+		/*
+		 * If paths have the same total cost, order them by startup cost.
+		 */
+		if (Abs(path1->startup_cost - path2->startup_cost) > fuzz)
+		{
+			if (path1->startup_cost < path2->startup_cost)
+				return -1;
+			else
+				return +1;
+		}
+	}
+	return 0;
+}
+
 /*
  * compare_path_fractional_costs
  *	  Return -1, 0, or +1 according as path1 is cheaper, the same cost,
@@ -215,29 +285,47 @@ add_path(RelOptInfo *parent_rel, Path *new_path)
 		bool		remove_old = false; /* unless new proves superior */
 		int			costcmp;
 
-		costcmp = compare_path_costs(new_path, old_path, TOTAL_COST);
+		/*
+		 * As of Postgres 7.5, we use fuzzy cost comparison to avoid wasting
+		 * cycles keeping paths that are really not significantly different
+		 * in cost.
+		 */
+		costcmp = compare_fuzzy_path_costs(new_path, old_path, TOTAL_COST);
 
 		/*
 		 * If the two paths compare differently for startup and total
 		 * cost, then we want to keep both, and we can skip the (much
 		 * slower) comparison of pathkeys.	If they compare the same,
 		 * proceed with the pathkeys comparison.  Note: this test relies
-		 * on the fact that compare_path_costs will only return 0 if both
-		 * costs are equal (and, therefore, there's no need to call it
-		 * twice in that case).
+		 * on the fact that compare_fuzzy_path_costs will only return 0 if
+		 * both costs are effectively equal (and, therefore, there's no need
+		 * to call it twice in that case).
 		 */
 		if (costcmp == 0 ||
-			costcmp == compare_path_costs(new_path, old_path,
-										  STARTUP_COST))
+			costcmp == compare_fuzzy_path_costs(new_path, old_path,
+												STARTUP_COST))
 		{
 			switch (compare_pathkeys(new_path->pathkeys, old_path->pathkeys))
 			{
 				case PATHKEYS_EQUAL:
 					if (costcmp < 0)
 						remove_old = true;		/* new dominates old */
+					else if (costcmp > 0)
+						accept_new = false;		/* old dominates new */
 					else
-						accept_new = false;		/* old equals or dominates
+					{
+						/*
+						 * Same pathkeys, and fuzzily the same cost, so
+						 * keep just one --- but we'll do an exact cost
+						 * comparison to decide which.
+						 */
+						if (compare_path_costs(new_path, old_path,
+											   TOTAL_COST) < 0)
+							remove_old = true; /* new dominates old */
+						else
+							accept_new = false;	/* old equals or dominates
 												 * new */
+					}
 					break;
 				case PATHKEYS_BETTER1:
 					if (costcmp <= 0)