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.
Comments
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.