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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);.
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 details and examples, see the "Custom Weighting Schemes" section in "Getting Started with Xapian".
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.
You can order documents by comparing a specified document value. Note that the comparison used compares the byte values in the value (i.e. it's a string sort ignoring locale), so 1 < 10 < 2. If you want to encode the value such that it sorts numerically, use Xapian::sortable_serialise() to encode values at index time - this works equally will on integers and floating point values:
Xapian::Document doc; doc.add_value(0, Xapian::sortable_serialise(price));
There are three methods which are used to specify how the value is used to sort, depending if/how you want relevance used in the ordering:
- Enquire::set_sort_by_value() specifies the relevance doesn't affect the ordering at all.
- Enquire::set_sort_by_value_then_relevance() specifies that relevance is used for ordering any groups of documents for which the value is the same.
- Enquire::set_sort_by_relevance_then_value() specifies that documents are ordered by relevance, and the value is only used to order groups of documents with identical relevance values (note: the weight has to be the same, not just the rounded percentage score). This method isn't very useful with the default BM25 weighting, which rarely assigns identical scores to different documents.
To allow more elaborate sorting schemes, Xapian allows you to provide a functor object subclassed from Xapian::KeyMaker which generates a sort key for each matching document which is under consideration. This is called at most once for each document, and then the generated sort keys are ordered by comparing byte values (i.e. with a string sort ignoring locale).
There's a standard subclass Xapian::MultiValueKeyMaker which allows sorting on more than one document value (so the first document value specified determines the order except among groups which have the same value, when the second document value specified is used, and so on).
Xapian::KeyMaker can also be subclassed to offer features such as "sort by geographical distance". A subclass could take a coordinate pair - e.g. (latitude, longitude) - for the user's location and sort results using coordinates stored in a document value so that the nearest results ranked highest.