Joins and the Optimizer - a question

I have a question about the optimizer (higher than Informix 7.11 since this doesn't seem to occur) when dealing with joins: Suppose I have two tables that I join by a field like Id (where a.Id = b.Id). I make an index on both tables on that field (Id), and I make them clustered. It would seem to me that the best method when pulling back a large data set would not be the hash join, but rather the sort-merge join, mainly because it would NOT have to do the sort, since the table is already sorted by that field. It would just have to do the merge. Thanks, Paul