Function assign_ordered_set_collations

src/backend/parser/parse_collate.c:923–948

* For ordered-set aggregates, it's somewhat unclear how best to proceed. * The spec-defined inverse distribution functions have only one sort column * and don't return collatable types, but this is clearly too restrictive in * the general case. Our solution is to consider that the aggregate's direct * arguments contribute normally to determination of the aggregate's own * collation, while ag

 * Otherwise this is much like the plain-aggregate case.
 */
static void
assign_ordered_set_collations(Aggref *aggref,
							  assign_collations_context *loccontext)
{
	bool		merge_sort_collations;
	ListCell   *lc;

	/* Merge sort collations to parent only if there can be only one */
	merge_sort_collations = (list_length(aggref->args) == 1 &&
							 get_func_variadictype(aggref->aggfnoid) == InvalidOid);

	/* Direct args, if any, are normal children of the Aggref node */
	(void) assign_collations_walker((Node *) aggref->aggdirectargs,
									loccontext);

	/* Process aggregated args appropriately */
	foreach(lc, aggref->args)
	{
		TargetEntry *tle = lfirst_node(TargetEntry, lc);

		if (merge_sort_collations)
			(void) assign_collations_walker((Node *) tle, loccontext);
		else
			assign_expr_collations(loccontext->pstate, (Node *) tle);
	}
}

/*
 * Hypothetical-set aggregates are even more special: per spec, we need to