Query never guarantee the results in specific order with out query ORDER BY.
You should use
select D,E,F from bucket where A=‘aa’ and B=‘bb’ ORDER BY C;
Still applies Rule #7 and query avoids sort in specific cases after analyzing (based on results from index did not alter the order).
Example: If query uses index1, no joins, no group by, no max_parallelism, query predicates A,B are equality predicate, …
You should do EXPLAIN and check Operator: Order at the end .
NOTE: If index1 is not online or query uses different index not specifying the query order by you may get results in different order. To avoid that you should specify ORDER BY if you expecting results in sorted order.