Tidy up join_search_one_level code

The code in join_search_one_level was a bit convoluted.  With a casual
glance, you might think that other_rels_list was being set to something
other than joinrels[1] when level == 2, however, joinrels[level - 1] is
joinrels[1] when level == 2, so nothing special needs to happen to set
other_rels_list.  Let's clean that up to avoid confusing anyone.

In passing, we may as well modernize the loop in
make_rels_by_clause_joins() and instead of passing in the ListCell to
start looping from, let's just pass in the index where to start from and
make use of for_each_from().  Ever since 1cff1b95a, Lists are arrays
under the hood. lnext() and list_head() both seem a little too linked-list
like.

Author: Alex Hsieh, David Rowley, Richard Guo
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CANWNU8x9P9aCXGF%3DaT-A_8mLTAT0LkcZ_ySYrGbcuHzMQw2-1g%40mail.gmail.com

src/backend/optimizer/path/joinrels.c

@@ -25,8 +25,8 @@
 
 static void make_rels_by_clause_joins(PlannerInfo *root,
 									  RelOptInfo *old_rel,
-									  List *other_rels_list,
+									  List *other_rels,
-									  ListCell *other_rels);
+									  int first_rel_idx);
 static void make_rels_by_clauseless_joins(PlannerInfo *root,
 										  RelOptInfo *old_rel,
 										  List *other_rels);
@@ -93,6 +93,8 @@ join_search_one_level(PlannerInfo *root, int level)
 		if (old_rel->joininfo != NIL || old_rel->has_eclass_joins ||
 			has_join_restriction(root, old_rel))
 		{
+			int			first_rel;
+
 			/*
 			 * There are join clauses or join order restrictions relevant to
 			 * this rel, so consider joins between this rel and (only) those
@@ -106,24 +108,12 @@ join_search_one_level(PlannerInfo *root, int level)
 			 * to each initial rel they don't already include but have a join
 			 * clause or restriction with.
 			 */
-			List	   *other_rels_list;
-			ListCell   *other_rels;
-
 			if (level == 2)		/* consider remaining initial rels */
-			{
+				first_rel = foreach_current_index(r) + 1;
-				other_rels_list = joinrels[level - 1];
+			else
-				other_rels = lnext(other_rels_list, r);
+				first_rel = 0;
-			}
-			else				/* consider all initial rels */
-			{
-				other_rels_list = joinrels[1];
-				other_rels = list_head(other_rels_list);
-			}
 
-			make_rels_by_clause_joins(root,
+			make_rels_by_clause_joins(root, old_rel, joinrels[1], first_rel);
-									  old_rel,
-									  other_rels_list,
-									  other_rels);
 		}
 		else
 		{
@@ -167,8 +157,7 @@ join_search_one_level(PlannerInfo *root, int level)
 		foreach(r, joinrels[k])
 		{
 			RelOptInfo *old_rel = (RelOptInfo *) lfirst(r);
-			List	   *other_rels_list;
+			int			first_rel;
-			ListCell   *other_rels;
 			ListCell   *r2;
 
 			/*
@@ -180,19 +169,12 @@ join_search_one_level(PlannerInfo *root, int level)
 				!has_join_restriction(root, old_rel))
 				continue;
 
-			if (k == other_level)
+			if (k == other_level)	/* only consider remaining rels */
-			{
+				first_rel = foreach_current_index(r) + 1;
-				/* only consider remaining rels */
-				other_rels_list = joinrels[k];
-				other_rels = lnext(other_rels_list, r);
-			}
 			else
-			{
+				first_rel = 0;
-				other_rels_list = joinrels[other_level];
-				other_rels = list_head(other_rels_list);
-			}
 
-			for_each_cell(r2, other_rels_list, other_rels)
+			for_each_from(r2, joinrels[other_level], first_rel)
 			{
 				RelOptInfo *new_rel = (RelOptInfo *) lfirst(r2);
 
@@ -286,9 +268,8 @@ join_search_one_level(PlannerInfo *root, int level)
  * automatically ensures that each new joinrel is only added to the list once.
  *
  * 'old_rel' is the relation entry for the relation to be joined
- * 'other_rels_list': a list containing the other
+ * 'other_rels': a list containing the other rels to be considered for joining
- * rels to be considered for joining
+ * 'first_rel_idx': the first rel to be considered in 'other_rels'
- * 'other_rels': the first cell to be considered
  *
  * Currently, this is only used with initial rels in other_rels, but it
  * will work for joining to joinrels too.
@@ -296,12 +277,12 @@ join_search_one_level(PlannerInfo *root, int level)
 static void
 make_rels_by_clause_joins(PlannerInfo *root,
 						  RelOptInfo *old_rel,
-						  List *other_rels_list,
+						  List *other_rels,
-						  ListCell *other_rels)
+						  int first_rel_idx)
 {
 	ListCell   *l;
 
-	for_each_cell(l, other_rels_list, other_rels)
+	for_each_from(l, other_rels, first_rel_idx)
 	{
 		RelOptInfo *other_rel = (RelOptInfo *) lfirst(l);