# Xapian Synonym Support

Table of contents

- Introduction
- Model
- QueryParser Integration
- Current Limitations
  - Explicit multi-word synonyms
  - Backend Support

# 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 that Xapian doesn't offer automated generation of the synonym dictionary.

# 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.

# QueryParser Integration

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

If `FLAG_SYNONYM` is passed to `QueryParser::parse_query()` then the 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 OR-ed with any listed synonyms, so `~truck` might expand to `truck OR lorry OR van`. A group of terms is handled in much the same way.

If a term to be synonym expanded will be stemmed by the 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.

If `FLAG_AUTO_SYNONYMS` is passed to `QueryParser::parse_query()` then the 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 `QueryParser::parse_query()` then the 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 QueryParser will look for further possible expansions starting with the term after the last term in the expanded group.

# 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 only supported by chert, flint and brass databases. They work with a single database or multiple databases (use 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.