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.
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.
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;