This really depends on your data's geometry. Just look at when SAS programmers are advised to use a merge or a format. Even hash-join vs merge-join really dep

This really depends on your data's geometry. Just look at when SAS programmers are advised to use a merge or a format. Even hash-join vs merge-join really depend on your data's cardinality (read: sizes), indices, etc. EDIT: Other comments also point out that there are non-general joins that are already NP-hard to optimize. You really want all the educated guesses you can get.