Synonyms

Introduction

Xapian provides support for storing a synonym dictionary, or thesaurus. This can be used by the xapian.QueryParser class to expand terms in user query strings, either automatically, or when requested by the user with an explicit synonym operator (~).

Note

Xapian doesn’t offer automated generation of the synonym dictionary.

Here is an example of search program with synonym functionality.


You can see the search results without ~ operator.

$ python2 code/python/search_synonyms.py db time
1: #065 Electric time piece with hands but without dial (no pendulum
2: #058 The "Empire" clock, to show the time at various longitudes,
3: #041 Frequency and time measuring instrument type TSA3436 by Venn
4: #056 Single sandglass in 4 pillared wood mount, running time 15 1
5: #043 Loughborough-Hayes automatic timing apparatus. Used by the R
6: #011 "Timetrunk" by Hines and Co., Glasgow (a sandglass for timin
7: #016 Copy of the gearing of the Byzantine sundial-calendar (1983-
8: #045 Master clock of the "Silent Electric" type made by the Magne
9: #018 Solar/Sidereal verge watch with epicyclic maintaining power
INFO:xapian.search:'time'[0:10] = 65 58 41 56 43 11 16 45 18

Notice the difference with the ~ operator with time where calendar is specified as its synonym.

$ python2 code/python/search_synonyms.py db ~time
1: #016 Copy of the gearing of the Byzantine sundial-calendar (1983-
2: #072 German Perpetual Calendar in gilt metal
3: #065 Electric time piece with hands but without dial (no pendulum
4: #068 Ornate brass Perpetual Calendar
5: #058 The "Empire" clock, to show the time at various longitudes,
6: #041 Frequency and time measuring instrument type TSA3436 by Venn
7: #056 Single sandglass in 4 pillared wood mount, running time 15 1
8: #043 Loughborough-Hayes automatic timing apparatus. Used by the R
9: #026 Sundial and compass with perpetual calendar and lunar circles
10: #036 Universal 'Tri-Compax' chronographic wrist watch
INFO:xapian.search:'~time'[0:10] = 16 72 65 68 58 41 56 43 26 36

Model

The model for the synonym dictionary is that a term or group of consecutive terms can have one or more synonym terms. A group of consecutive terms is specified in the dictionary by simply joining them with a single space between each one.

If a term to be synonym expanded will be stemmed by the xapian.QueryParser, then synonyms will be checked for the unstemmed form first, and then for the stemmed form, so you can provide different synonyms for particular unstemmed forms if you want to.

Todo

Discuss interactions with stemming (ie, should the input and/or output values in the synonym table be stemmed).

Adding Synonyms

The synonyms can be added by the xapian.WritableDatabase.add_synonym(). In the following example calender is specified as a synonym for time. Users may similarly write a loop to load all the synonyms from a dictionary file.

    db.add_synonym("time", "calendar")

QueryParser Integration

In order for any of the synonym features of the QueryParser to work, you must call xapian.QueryParser.set_database() to specify the database to use.

    queryparser.set_database(db)

If FLAG_SYNONYM is passed to xapian.QueryParser.parse_query() then the xapian.QueryParser will recognise ~ in front of a term as indicating a request for synonym expansion.

If FLAG_LOVEHATE is also specified, you can use + and - before the ~ to indicate that you love or hate the synonym expanded expression.

A synonym-expanded term becomes the term itself OP_SYNONYM-ed with any listed synonyms, so ~truck might expand to truck SYNONYM lorry SYNONYM van. A group of terms is handled in much the same way.

If FLAG_AUTO_SYNONYMS is passed to xapian.QueryParser.parse_query() then the :xapian-class:` QueryParser` will automatically expand any term which has synonyms, unless the term is in a phrase or similar.

If FLAG_AUTO_MULTIWORD_SYNONYMS is passed to xapian.QueryParser.parse_query() then the :xapian-class:` QueryParser` will look at groups of terms separated only by whitespace and try to expand them as term groups. This is done in a “greedy” fashion, so the first term which can start a group is expanded first, and the longest group starting with that term is expanded. After expansion, the :xapian-class:` QueryParser` will look for further possible expansions starting with the term after the last term in the expanded group.

OP_SYNONYM

Todo

Query.OP_SYNONYM, and how that relates to synonym expansion.

Current Limitations

Explicit multi-word synonyms

There ought to be a way to explicitly request expansion of multi-term synonyms, probably with the syntax ~"stock market". This hasn’t been implemented yet though.

Backend Support

Currently synonyms are supported by the chert and glass databases. They work with a single database or multiple databases (use xapian.Database.add_database() as usual). We’ve no plans to support them for the InMemory backend, but we do intend to support them for the remote backend in the future.