# Xapian Faceting Support

Table of contents

- Introduction
- How to use Faceting
  - Indexing
  - Searching
    - Finding Facets
    - Restricting by Facet Values

# Introduction

Xapian provides functionality which allows you to dynamically generate complete lists of category values which feature in matching documents. There are numerous potential uses this can be put to, but a common one is to offer the user the ability to narrow down their search by filtering it to only include documents with a particular value of a particular category. This is often referred to as `faceted search`.

You may have many multiple facets (for example colour, manufacturer, product type) so Xapian allows you to handle multiple facets at once.

# How to use Faceting

## Indexing

When indexing a document, you need to add each facet in a different numbered value slot. As described elsewhere in the documentation, each Xapian document has a set of "value slots", each of which is addressed by a number, and can contain a value which is an arbitrary string.

The `Xapian::Document::add_value()` method can be used to put values into a particular slot. So, if you had a database of books, you might put "price" facet values in slot 0, say (serialised to strings using `Xapian::sortable_serialise`, or some similar function), "author" facet values in slot 1, "publisher" facet values in slot 2 and "publication type" (eg, hardback, softback, etc) values in slot 3.

## Searching

### Finding Facets

At search time, for each facet you want to consider, you need to get a count of the number of times each facet value occurs in each slot; for the example above, if you wanted to get facets for "price", "author" and "publication type" you'd want to get the counts from slots 0, 1 and 3.

This can be done by calling `Xapian::Enquire::add_matchspy()` with a pointer to a `Xapian::ValueCountMatchSpy` object for each value slot you want to get facet counts for, like so:

    Xapian::ValueCountMatchSpy spy0(0);
    Xapian::ValueCountMatchSpy spy1(1);
    Xapian::ValueCountMatchSpy spy3(3);

    Xapian::Enquire enq(db);
    enq.add_matchspy(&spy0);
    enq.add_matchspy(&spy1);
    enq.add_matchspy(&spy3);

    enq.set_query(query);

    Xapian::MSet mset = enq.get_mset(0, 10, 10000);

The `10000` in the call to `get_mset()` tells Xapian to check at least 10000 documents, so the MatchSpy objects will be passed at least 10000 documents to tally facet information from (unless fewer than 10000 documents match the query, in which case they will see all of them). Setting this to `db.get_doccount()` will make the facet counts exact, but Xapian will have to do more work for most queries so searches will be slower.

The `spy` objects now contain the facet information. You can find out how many documents they looked at by calling `spy0.get_total()`. (All the spies will have looked at the same number of documents.) You can read the values from, say, `spy0` like this:

    Xapian::TermIterator i;
    for (i = spy0.values_begin(); i != spy0.values_end(); ++i) {
        cout << *i << ": " << i.get_termfreq() << endl;
    }

### Restricting by Facet Values

If you're using the facets to offer the user choices for narrowing down their search results, you then need to be able to apply a suitable filter.

For a single value, you could use `Xapian::Query::OP_VALUE_RANGE` with the same start and end, or `Xapian::MatchDecider`, but it's probably most efficient to also index the categories as suitably prefixed boolean terms and use those for filtering.