Xapian posting lists in memory mode

Dear Members of Xapian Project,

    Sorry for troubling you this time. This is Miao LIU, a postgraduate student working on information retrieval system related areas.
I have been using Xapian (https://xapian.org) for quite a few months as the database and the searching engine in my IR system. I am currently facing
a tough problem that I'd like to put all my posting lists in memory of my high-end machine. I went through all related tutorials, documents and
materials of the Xapian but I found nothing except a class called "InMemoryDatabase" and a related issue (https://trac.xapian.org/ticket/59#no1) without
updating in recent years. As for the class "InMemoryDatabase", I found pages of tutorials then tried and drew the conclusion that the "InMemoryDatabase"
unfortunately can not out-performs the general DiskDatabase mode.

    So, could you give me an illustration about how Xapian currently supports in-memory postlists (if any) or the reason why the community gives up supporting
such a advanced feature. Meanwhile I am checking the source code of Xapian and trying to implement the feature mentioned above. I am glad to receive
any advise.

    Looking forward to your early reply.

Best Regards,
Miao LIU