merge-joinを明示的に実行する方法
はありません、とMLで教えてもらいました。
As long as your query consists entirely of equality filters and does not have any sort orders, it will not require a composite index. If your query only filters on a single property it will be executed using a built-in index. If your query filters on more than one property it will be executed using a merge-join across built-in indices.
（超訳：複数のプロパティを使うクエリで、コンポジットインデックスを作らない場合は、自動的にmerge-joinになる）
All of this said, I wouldn't recommend thinking about your queries in terms of the underlying query plan, since those are implementation details and liable to change. Avoiding composite indices will make your writes faster and save on storage, so that's certainly worthwhile if those are pain points in your application, but beyond that there isn't really much reason to target specific types of query plans.
（「アプリが重くなるようなコンポジットインデックスはできるだけ作るべきでない」ことは確かだが、それ以外の内部的なクエリプランについては変更される可能性もあるからあまり意識しない方がいい）
