The document boost is a nice feature but since it was removed from lucene 4.0, the way it works in elasticsearch is by adding fields boosts to each field, multiplying it with the original field boost. Here is the interesting commit: c60f204#diff-7117c679a1ca0d5002c0c9b9ef8bad16 . That is not exactly how the document boost should work, it has downsides and the same result can be obtained using function_score.

For the above reasons we are going to deprecate the document boost.