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 1cff1b95a 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 1cff1b95a.

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 1cff1b95a, and are unused as of
cc99baa43.  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

3 files changed, 5 insertions, 7 deletions

diff --git a/src/backend/utils/adt/jsonpath_gram.y b/src/backend/utils/adt/jsonpath_gram.y
index f87db8ccf6b..9947fedfc19 100644
--- a/src/backend/utils/adt/jsonpath_gram.y
+++ b/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);
 
diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 076c3c019ff..9db584169fc 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -8123,7 +8123,7 @@ 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)
 				{
@@ -8132,12 +8132,11 @@ 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, ')');
@@ -8148,12 +8147,11 @@ 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, ')');
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 8f06e8212d7..b3825633c0a 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/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);