F.12. dict_xsyn
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## F.12. dict_xsyn

`dict_xsyn` (Extended Synonym Dictionary) is an example of an add-on dictionary template for full-text search. This dictionary type replaces words with groups of their synonyms, and so makes it possible to search for a word using any of its synonyms.

### F.12.1. Configuration

A `dict_xsyn` dictionary accepts the following options:

- `matchorig` controls whether the original word is accepted by the dictionary. Default is `true`.

- `matchsynonyms` controls whether the synonyms are accepted by the dictionary. Default is `false`.

- `keeporig` controls whether the original word is included in the dictionary's output. Default is `true`.

- `keepsynonyms` controls whether the synonyms are included in the dictionary's output. Default is `true`.

- `rules` is the base name of the file containing the list of synonyms. This file must be stored in `$SHAREDIR/tsearch_data/` (where `$SHAREDIR` means the PostgreSQL installation's shared-data directory). Its name must end in `.rules` (which is not to be included in the `rules` parameter).

The rules file has the following format:

- Each line represents a group of synonyms for a single word, which is given first on the line. Synonyms are separated by whitespace, thus:

      word syn1 syn2 syn3

- The sharp (`#`) sign is a comment delimiter. It may appear at any position in a line. The rest of the line will be skipped.

Look at `xsyn_sample.rules`, which is installed in `$SHAREDIR/tsearch_data/`, for an example.

### F.12.2. Usage

Installing the `dict_xsyn` extension creates a text search template `xsyn_template` and a dictionary `xsyn` based on it, with default parameters. You can alter the parameters, for example

    mydb# ALTER TEXT SEARCH DICTIONARY xsyn (RULES='my_rules', KEEPORIG=false);
    ALTER TEXT SEARCH DICTIONARY

or create new dictionaries based on the template.

To test the dictionary, you can try

    mydb=# SELECT ts_lexize('xsyn', 'word');
          ts_lexize
    -----------------------
     {syn1,syn2,syn3}

    mydb# ALTER TEXT SEARCH DICTIONARY xsyn (RULES='my_rules', KEEPORIG=true);
    ALTER TEXT SEARCH DICTIONARY

    mydb=# SELECT ts_lexize('xsyn', 'word');
          ts_lexize
    -----------------------
     {word,syn1,syn2,syn3}

    mydb# ALTER TEXT SEARCH DICTIONARY xsyn (RULES='my_rules', KEEPORIG=false, MATCHSYNONYMS=true);
    ALTER TEXT SEARCH DICTIONARY

    mydb=# SELECT ts_lexize('xsyn', 'syn1');
          ts_lexize
    -----------------------
     {syn1,syn2,syn3}

    mydb# ALTER TEXT SEARCH DICTIONARY xsyn (RULES='my_rules', KEEPORIG=true, MATCHORIG=false, KEEPSYNONYMS=false);
    ALTER TEXT SEARCH DICTIONARY

    mydb=# SELECT ts_lexize('xsyn', 'syn1');
          ts_lexize
    -----------------------
     {word}

Real-world usage will involve including it in a text search configuration as described in Chapter 12. That might look like this:

    ALTER TEXT SEARCH CONFIGURATION english
        ALTER MAPPING FOR word, asciiword WITH xsyn, english_stem;

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