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unaccent is a text search dictionary that removes accents (diacritic signs) from lexemes. It's a filtering dictionary, which means its output is always passed to the next dictionary (if any), unlike the normal behavior of dictionaries. This allows accent-insensitive processing for full text search.
The current implementation of unaccent cannot be used as a normalizing dictionary for the thesaurus dictionary.
An unaccent dictionary accepts the following options:
RULES is the base name of the file containing the list of translation rules. 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 pair, consisting of a character with accent followed by a character without accent. The first is translated into the second. For example,
À A Á A Â A Ã A Ä A Å A Æ A
A more complete example, which is directly useful for most European languages, can be found in unaccent.rules, which is installed in $SHAREDIR/tsearch_data/ when the unaccent module is installed.
Running the installation script unaccent.sql creates a text search template unaccent and a dictionary unaccent based on it. The unaccent dictionary has the default parameter setting RULES='unaccent', which makes it immediately usable with the standard unaccent.rules file. If you wish, you can alter the parameter, for example
mydb=# ALTER TEXT SEARCH DICTIONARY unaccent (RULES='my_rules');
or create new dictionaries based on the template.
To test the dictionary, you can try:
mydb=# select ts_lexize('unaccent','Hôtel');
ts_lexize
-----------
{Hotel}
(1 row)
Here is an example showing how to insert the unaccent dictionary into a text search configuration:
mydb=# CREATE TEXT SEARCH CONFIGURATION fr ( COPY = french );
mydb=# ALTER TEXT SEARCH CONFIGURATION fr
ALTER MAPPING FOR hword, hword_part, word
WITH unaccent, french_stem;
mydb=# select to_tsvector('fr','Hôtels de la Mer');
to_tsvector
-------------------
'hotel':1 'mer':4
(1 row)
mydb=# select to_tsvector('fr','Hôtel de la Mer') @@ to_tsquery('fr','Hotels');
?column?
----------
t
(1 row)
mydb=# select ts_headline('fr','Hôtel de la Mer',to_tsquery('fr','Hotels'));
ts_headline
------------------------
<b>Hôtel</b> de la Mer
(1 row)
The unaccent() function removes accents (diacritic signs) from
a given string. Basically, it's a wrapper around the
unaccent dictionary, but it can be used outside normal
text search contexts.
unaccent([dictionary, ] string) returns text
For example:
SELECT unaccent('unaccent', 'Hôtel');
SELECT unaccent('Hôtel');