Make better use of the new List implementation in a couple of places

In nodeAppend.c and nodeMergeAppend.c there were some foreach loops which
looped over the list of subplans and only performed any work if the
subplan index was found in a Bitmapset.  With the old linked list
implementation of List, this form made sense as accessing the Nth list
element was O(N).  However, thanks to 1cff1b95 we now have array-based
lists, so accessing the Nth element has become O(1).

Here we make the most of the O(1) lookups and just loop over the set
members of the Bitmapset with bms_next_member().  This performs slightly
better when a small number of the list items are in the Bitmapset.  Micro
benchmarks show that when the Bitmapset contains all or most of the list
items then the new code is ever so slightly slower.  In practice, the cost
is so small that it's drowned out by various other things such as locking
the relations belonging to each subplan, etc.

The primary goal here is to leave better code examples around which benefit
better from the new list implementation.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAKJS1f8ZcsLVgkF4wOfRyMYTcPgLFiUAOedFC+U2vK_aFZk-BA@mail.gmail.com

src/backend/executor/nodeAppend.c

@@ -107,7 +107,6 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 	int			firstvalid;
 	int			i,
 				j;
-	ListCell   *lc;
 
 	/* check for unsupported flags */
 	Assert(!(eflags & EXEC_FLAG_MARK));
@@ -211,24 +210,20 @@ ExecInitAppend(Append *node, EState *estate, int eflags)
 	 *
 	 * While at it, find out the first valid partial plan.
 	 */
-	j = i = 0;
+	j = 0;
 	firstvalid = nplans;
-	foreach(lc, node->appendplans)
+	i = -1;
+	while ((i = bms_next_member(validsubplans, i)) >= 0)
 	{
-		if (bms_is_member(i, validsubplans))
-		{
-			Plan	   *initNode = (Plan *) lfirst(lc);
+		Plan	   *initNode = (Plan *) list_nth(node->appendplans, i);
 
-			/*
-			 * Record the lowest appendplans index which is a valid partial
-			 * plan.
-			 */
-			if (i >= node->first_partial_plan && j < firstvalid)
-				firstvalid = j;
+		/*
+		 * Record the lowest appendplans index which is a valid partial plan.
+		 */
+		if (i >= node->first_partial_plan && j < firstvalid)
+			firstvalid = j;
 
-			appendplanstates[j++] = ExecInitNode(initNode, estate, eflags);
-		}
-		i++;
+		appendplanstates[j++] = ExecInitNode(initNode, estate, eflags);
 	}
 
 	appendstate->as_first_partial_plan = firstvalid;

src/backend/executor/nodeMergeAppend.c

@@ -70,7 +70,6 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 	int			nplans;
 	int			i,
 				j;
-	ListCell   *lc;
 
 	/* check for unsupported flags */
 	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
@@ -177,16 +176,13 @@ ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
 	 * call ExecInitNode on each of the valid plans to be executed and save
 	 * the results into the mergeplanstates array.
 	 */
-	j = i = 0;
-	foreach(lc, node->mergeplans)
+	j = 0;
+	i = -1;
+	while ((i = bms_next_member(validsubplans, i)) >= 0)
 	{
-		if (bms_is_member(i, validsubplans))
-		{
-			Plan	   *initNode = (Plan *) lfirst(lc);
+		Plan	   *initNode = (Plan *) list_nth(node->mergeplans, i);
 
-			mergeplanstates[j++] = ExecInitNode(initNode, estate, eflags);
-		}
-		i++;
+		mergeplanstates[j++] = ExecInitNode(initNode, estate, eflags);
 	}
 
 	mergestate->ps.ps_ProjInfo = NULL;