[dapperdrake]

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.