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;