Introduction

By default, Xapian orders search results by decreasing relevance score. However, it also allows results to be ordered by other criteria, or a mixture of other criteria and relevance score.

If two or more results compare equal by the sorting criteria, then their order is decided by their document ids. By default, the document ids sort in ascending order (so a lower document id is "better"), but this can be set to descending using enquire.set_docid_order(enquire.DESCENDING);. If you have no preference, you can tell Xapian to use whatever order is most efficient using enquire.set_docid_order(enquire.DONT_CARE);.

Sorting by Relevance

The BM25 weighting formula which Xapian uses by default has a number of parameters. We have picked some default parameter values which do a good job in general. The optimal values of these parameters depend on the data being indexed and the type of queries being run, so you may be able to improve the effectiveness of your search system by adjusting these values, but it's a fiddly process to tune them so people tend not to bother.

See the BM25 documentation for more details of BM25.

The other included weighting schemes are TradWeight and BoolWeight.

TradWeight implements the original probabilistic weighting formula, which is essentially a special case of BM25 (it's BM25 with k2 = 0, k3 = 0, b = 1, and min_normlen = 0, except that the weights are scaled by a constant factor).

BoolWeight assigns a weight of 0 to all documents, so the ordering is determined solely by other factors.

You can also implement your own weighting scheme, provided it can be expressed in the form of a sum over the matching terms, plus an extra term which depends on term-independent statistics (such as the normalised document length).

For example, here's an implementation of "coordinate matching" - each matching term scores one point:

class CoordinateWeight : public Xapian::Weight {
  public:
    CoordinateWeight * clone() const { return new CoordinateWeight; }
    CoordinateWeight() { }
    ~CoordinateWeight() { }

    std::string name() const { return "Coord"; }
    std::string serialise() const { return ""; }
    CoordinateWeight * unserialise(const std::string &) const {
        return new CoordinateWeight;
    }

    Xapian::weight get_sumpart(Xapian::termcount, Xapian::doclength) const {
        return 1;
    }
    Xapian::weight get_maxpart() const { return 1; }

    Xapian::weight get_sumextra(Xapian::doclength) const { return 0; }
    Xapian::weight get_maxextra() const { return 0; }

    bool get_sumpart_needs_doclength() const { return false; }
};

Sorting by Other Properties

If you want to offer a "sort by date" feature, and can arrange for documents to be indexed in date order (or a close-enough approximation), then you can implement a very efficient "sort by date" feature by using a boolean search (i.e. call enquire.set_weighting_scheme(Xapian::BoolWeight());) with enquire.set_docid_order(Xapian::Enquire::DESCENDING); (for newest first) or enquire.set_docid_order(Xapian::Enquire::ASCENDING); (for oldest first). There's no inherent reason why this technique can't be used for sorting by something other than date, but it's usually much easier to arrange for new documents to arrive in date order than in other orders.

Xapian::Document doc;
doc.add_value(0, Xapian::sortable_serialise(price));