postgresql_distinct_on.c

	/*
	 * If the user writes both DISTINCT ON and ORDER BY, adopt the sorting
	 * semantics from ORDER BY items that match DISTINCT ON items, and also
	 * adopt their column sort order.  We insist that the distinctClause and
	 * sortClause match, so throw error if we find the need to add any more
	 * distinctClause items after we've skipped an ORDER BY item that wasn't
	 * in DISTINCT ON.
	 */
	skipped_sortitem = false;
	foreach(lc, sortClause)
	{
		SortGroupClause *scl = (SortGroupClause *) lfirst(lc);

		if (list_member_int(sortgrouprefs, scl->tleSortGroupRef))
		{
			if (skipped_sortitem)
				ereport(ERROR,
						(errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
						 errmsg("SELECT DISTINCT ON expressions must match initial ORDER BY expressions"),
						 parser_errposition(pstate,
								  get_matching_location(scl->tleSortGroupRef,
														sortgrouprefs,
														distinctlist))));
			else
				result = lappend(result, copyObject(scl));
		}
		else
			skipped_sortitem = true;
	}