Function flatten_grouping_sets

src/backend/parser/parse_clause.c:2344–2432

------------------------------------------------------------------------- * Flatten out parenthesized sublists in grouping lists, and some cases * of nested grouping sets. * * Inside a grouping set (ROLLUP, CUBE, or GROUPING SETS), we expect the * content to be nested no more than 2 deep: i.e. ROLLUP((a,b),(c,d)) is * ok, but ROLLUP((a,(b,c)),d) is flattened to ((a,b,c),d), which we then *

 *-------------------------------------------------------------------------
 */
static Node *
flatten_grouping_sets(Node *expr, bool toplevel, bool *hasGroupingSets)
{
	/* just in case of pathological input */
	check_stack_depth();

	if (expr == (Node *) NIL)
		return (Node *) NIL;

	switch (expr->type)
	{
		case T_RowExpr:
			{
				RowExpr    *r = (RowExpr *) expr;

				if (r->row_format == COERCE_IMPLICIT_CAST)
					return flatten_grouping_sets((Node *) r->args,
												 false, NULL);
			}
			break;
		case T_GroupingSet:
			{
				GroupingSet *gset = (GroupingSet *) expr;
				ListCell   *l2;
				List	   *result_set = NIL;

				if (hasGroupingSets)
					*hasGroupingSets = true;

				/*
				 * at the top level, we skip over all empty grouping sets; the
				 * caller can supply the canonical GROUP BY () if nothing is
				 * left.
				 */

				if (toplevel && gset->kind == GROUPING_SET_EMPTY)
					return (Node *) NIL;

				foreach(l2, gset->content)
				{
					Node	   *n1 = lfirst(l2);
					Node	   *n2 = flatten_grouping_sets(n1, false, NULL);

					if (IsA(n1, GroupingSet) &&
						((GroupingSet *) n1)->kind == GROUPING_SET_SETS)
						result_set = list_concat(result_set, (List *) n2);
					else
						result_set = lappend(result_set, n2);
				}

				/*
				 * At top level, keep the grouping set node; but if we're in a
				 * nested grouping set, then we need to concat the flattened
				 * result into the outer list if it's simply nested.
				 */

				if (toplevel || (gset->kind != GROUPING_SET_SETS))
				{