About xapian serialization on float/double variables

[Miao LIU]

Dear Members of Xapian Project,

    Sorry for troubling you this time. It can be witnessed that xapian will store Document values with serialization approach when given value types meet float/double.

    Such an approach is deployed on sort_key related fields as well, where the xapian requires KeyMaker::operator() must return an serialized float/dobule variable. Then heap sort comes and ranks the vector<MSetItem> items (multimatch.cc MultiMatch::get_mset()) by comparing serialized sort_keys (std::string) straightforwardly according to <IEEE-754 doubles>. Subsequently sort_keys will be unserialized when user needs to read its real float/double values during iterations of result MSet.

    Obviously, serialization and unserialization are time-consuming operations. Compared with defining and using sort_key as float/double type directly, it is complicated to understand benefits of such serialization above in both performance and coding aspects.

    It will be very kind of you if you could give a short illustration. Looking forward to your early reply.

Best Regards,
Miao LIU