I'm skeptical in adding a mass-delete functionality on the FullTextSession as I don't see a good use for it. On the other hand I understand the good of having such a thing at the SPI level, as Martin's need is legit but as he stated in the description the goal is to provide a more efficient alternative than issuing a list of delete operations.

If we expose a query of type org.apache.lucene.Query on a method which needs to be sent to the backend, this needs to be serialized as some backends require this, and I haven't found a reliable and maintainable strategy to serialize these object instances. Other frameworks refrain from exposing such a type and will accept for example a query string (like the REST based servers obviously are STRING and URL based so that's easy to serialize.

Maybe, maybe not. I think we discussed the need for "serializing" Lucene queries in other contexts as well. Since there is an approach which does not use query serialization (query by projection and purge), we don't have to solve this problem right now. Once we have a proper solution it would be a matter of changing some internal parts.

I'm not understanding your position here. So what value would we be adding? Nobody needs this on public API, and Martin's request is having the goal to do a multiple-id delete in a more efficient way.