Add for_each_from, to simplify loops starting from non-first list cells.

We have a dozen or so places that need to iterate over all but the
first cell of a List.  Prior to v13 this was typically written as
	for_each_cell(lc, lnext(list_head(list)))
Commit 1cff1b95 changed these to
	for_each_cell(lc, list, list_second_cell(list))
This patch introduces a new macro for_each_from() which expresses
the start point as a list index, allowing these to be written as
	for_each_from(lc, list, 1)
This is marginally more efficient, since ForEachState.i can be
initialized directly instead of backing into it from a ListCell
address.  It also seems clearer and less typo-prone.

Some of the remaining uses of for_each_cell() look like they could
profitably be changed to for_each_from(), but here I confined myself
to changing uses of list_second_cell().

Also, fix for_each_cell_setup() and for_both_cell_setup() to
const-ify their arguments; that's a simple oversight in 1cff1b95.

Back-patch into v13, on the grounds that (1) the const-ification
is a minor bug fix, and (2) it's better for back-patching purposes
if we only have two ways to write these loops rather than three.

In HEAD, also remove list_third_cell() and list_fourth_cell(),
which were also introduced in 1cff1b95, and are unused as of
cc99baa4.  It seems unlikely that any third-party code would
have started to use them already; anyone who has can be directed
to list_nth_cell instead.

Discussion: https://postgr.es/m/CAApHDvpo1zj9KhEpU2cCRZfSM3Q6XGdhzuAS2v79PH7WJBkYVA@mail.gmail.com

src/backend/commands/tablecmds.c

@@ -5732,7 +5732,7 @@ ATCheckPartitionsNotInUse(Relation rel, LOCKMODE lockmode)
 
 		inh = find_all_inheritors(RelationGetRelid(rel), lockmode, NULL);
 		/* first element is the parent rel; must ignore it */
-		for_each_cell(cell, inh, list_second_cell(inh))
+		for_each_from(cell, inh, 1)
 		{
 			Relation	childrel;
 

src/backend/nodes/nodeFuncs.c

@@ -441,7 +441,7 @@ exprTypmod(const Node *expr)
 				typmod = exprTypmod((Node *) linitial(cexpr->args));
 				if (typmod < 0)
 					return -1;	/* no point in trying harder */
-				for_each_cell(arg, cexpr->args, list_second_cell(cexpr->args))
+				for_each_from(arg, cexpr->args, 1)
 				{
 					Node	   *e = (Node *) lfirst(arg);
 
@@ -469,7 +469,7 @@ exprTypmod(const Node *expr)
 				typmod = exprTypmod((Node *) linitial(mexpr->args));
 				if (typmod < 0)
 					return -1;	/* no point in trying harder */
-				for_each_cell(arg, mexpr->args, list_second_cell(mexpr->args))
+				for_each_from(arg, mexpr->args, 1)
 				{
 					Node	   *e = (Node *) lfirst(arg);
 

src/backend/optimizer/plan/createplan.c

@@ -2261,7 +2261,7 @@ create_groupingsets_plan(PlannerInfo *root, GroupingSetsPath *best_path)
 	{
 		bool		is_first_sort = ((RollupData *) linitial(rollups))->is_hashed;
 
-		for_each_cell(lc, rollups, list_second_cell(rollups))
+		for_each_from(lc, rollups, 1)
 		{
 			RollupData *rollup = lfirst(lc);
 			AttrNumber *new_grpColIdx;

src/backend/optimizer/plan/planner.c

@@ -4430,7 +4430,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 			 * below, must use the same condition.
 			 */
 			i = 0;
-			for_each_cell(lc, gd->rollups, list_second_cell(gd->rollups))
+			for_each_from(lc, gd->rollups, 1)
 			{
 				RollupData *rollup = lfirst_node(RollupData, lc);
 
@@ -4464,7 +4464,7 @@ consider_groupingsets_paths(PlannerInfo *root,
 				rollups = list_make1(linitial(gd->rollups));
 
 				i = 0;
-				for_each_cell(lc, gd->rollups, list_second_cell(gd->rollups))
+				for_each_from(lc, gd->rollups, 1)
 				{
 					RollupData *rollup = lfirst_node(RollupData, lc);
 

src/backend/parser/parse_agg.c

@@ -1083,7 +1083,7 @@ parseCheckAggregates(ParseState *pstate, Query *qry)
 
 		if (gset_common)
 		{
-			for_each_cell(l, gsets, list_second_cell(gsets))
+			for_each_from(l, gsets, 1)
 			{
 				gset_common = list_intersection_int(gset_common, lfirst(l));
 				if (!gset_common)
@@ -1774,7 +1774,7 @@ expand_grouping_sets(List *groupingSets, int limit)
 		result = lappend(result, list_union_int(NIL, (List *) lfirst(lc)));
 	}
 
-	for_each_cell(lc, expanded_groups, list_second_cell(expanded_groups))
+	for_each_from(lc, expanded_groups, 1)
 	{
 		List	   *p = lfirst(lc);
 		List	   *new_result = NIL;

src/backend/utils/adt/jsonpath_gram.y

@@ -441,7 +441,7 @@ makeItemList(List *list)
 	while (end->next)
 		end = end->next;
 
-	for_each_cell(cell, list, list_second_cell(list))
+	for_each_from(cell, list, 1)
 	{
 		JsonPathParseItem *c = (JsonPathParseItem *) lfirst(cell);
 

src/backend/utils/adt/ruleutils.c

@@ -8113,7 +8113,7 @@ get_rule_expr(Node *node, deparse_context *context,
 			{
 				BoolExpr   *expr = (BoolExpr *) node;
 				Node	   *first_arg = linitial(expr->args);
-				ListCell   *arg = list_second_cell(expr->args);
+				ListCell   *arg;
 
 				switch (expr->boolop)
 				{
@@ -8122,12 +8122,11 @@ get_rule_expr(Node *node, deparse_context *context,
 							appendStringInfoChar(buf, '(');
 						get_rule_expr_paren(first_arg, context,
 											false, node);
-						while (arg)
+						for_each_from(arg, expr->args, 1)
 						{
 							appendStringInfoString(buf, " AND ");
 							get_rule_expr_paren((Node *) lfirst(arg), context,
 												false, node);
-							arg = lnext(expr->args, arg);
 						}
 						if (!PRETTY_PAREN(context))
 							appendStringInfoChar(buf, ')');
@@ -8138,12 +8137,11 @@ get_rule_expr(Node *node, deparse_context *context,
 							appendStringInfoChar(buf, '(');
 						get_rule_expr_paren(first_arg, context,
 											false, node);
-						while (arg)
+						for_each_from(arg, expr->args, 1)
 						{
 							appendStringInfoString(buf, " OR ");
 							get_rule_expr_paren((Node *) lfirst(arg), context,
 												false, node);
-							arg = lnext(expr->args, arg);
 						}
 						if (!PRETTY_PAREN(context))
 							appendStringInfoChar(buf, ')');

src/backend/utils/adt/selfuncs.c

@@ -3519,7 +3519,7 @@ estimate_num_groups(PlannerInfo *root, List *groupExprs, double input_rows,
 		 * for remaining Vars on other rels.
 		 */
 		relvarinfos = lappend(relvarinfos, varinfo1);
-		for_each_cell(l, varinfos, list_second_cell(varinfos))
+		for_each_from(l, varinfos, 1)
 		{
 			GroupVarInfo *varinfo2 = (GroupVarInfo *) lfirst(l);
 

src/include/nodes/pg_list.h

@@ -144,26 +144,6 @@ list_second_cell(const List *l)
 		return NULL;
 }
 
-/* Fetch address of list's third cell, if it has one, else NULL */
-static inline ListCell *
-list_third_cell(const List *l)
-{
-	if (l && l->length >= 3)
-		return &l->elements[2];
-	else
-		return NULL;
-}
-
-/* Fetch address of list's fourth cell, if it has one, else NULL */
-static inline ListCell *
-list_fourth_cell(const List *l)
-{
-	if (l && l->length >= 4)
-		return &l->elements[3];
-	else
-		return NULL;
-}
-
 /* Fetch list's length */
 static inline int
 list_length(const List *l)
@@ -389,6 +369,32 @@ lnext(const List *l, const ListCell *c)
  */
 #define foreach_current_index(cell)  (cell##__state.i)
 
+/*
+ * for_each_from -
+ *	  Like foreach(), but start from the N'th (zero-based) list element,
+ *	  not necessarily the first one.
+ *
+ * It's okay for N to exceed the list length, but not for it to be negative.
+ *
+ * The caveats for foreach() apply equally here.
+ */
+#define for_each_from(cell, lst, N)	\
+	for (ForEachState cell##__state = for_each_from_setup(lst, N); \
+		 (cell##__state.l != NIL && \
+		  cell##__state.i < cell##__state.l->length) ? \
+		 (cell = &cell##__state.l->elements[cell##__state.i], true) : \
+		 (cell = NULL, false); \
+		 cell##__state.i++)
+
+static inline ForEachState
+for_each_from_setup(const List *lst, int N)
+{
+	ForEachState r = {lst, N};
+
+	Assert(N >= 0);
+	return r;
+}
+
 /*
  * for_each_cell -
  *	  a convenience macro which loops through a list starting from a
@@ -405,7 +411,7 @@ lnext(const List *l, const ListCell *c)
 		 cell##__state.i++)
 
 static inline ForEachState
-for_each_cell_setup(List *lst, ListCell *initcell)
+for_each_cell_setup(const List *lst, const ListCell *initcell)
 {
 	ForEachState r = {lst,
 	initcell ? list_cell_number(lst, initcell) : list_length(lst)};
@@ -456,8 +462,8 @@ for_each_cell_setup(List *lst, ListCell *initcell)
 		 cell1##__state.i1++, cell1##__state.i2++)
 
 static inline ForBothCellState
-for_both_cell_setup(List *list1, ListCell *initcell1,
-					List *list2, ListCell *initcell2)
+for_both_cell_setup(const List *list1, const ListCell *initcell1,
+					const List *list2, const ListCell *initcell2)
 {
 	ForBothCellState r = {list1, list2,
 		initcell1 ? list_cell_number(list1, initcell1) : list_length(list1),