Document filtering dictionaries in textsearch.sgml.
While at it, copy-edit the description of prefix-match marker support in synonym dictionaries, and clarify the description of the default unaccent dictionary a bit more.
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Tom Lane
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<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.58 2010/08/20 13:59:45 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.59 2010/08/25 21:42:55 tgl Exp $ -->
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<chapter id="textsearch">
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<title>Full Text Search</title>
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@ -112,7 +112,7 @@
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as a sorted array of normalized lexemes. Along with the lexemes it is
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often desirable to store positional information to use for
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<firstterm>proximity ranking</firstterm>, so that a document that
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contains a more <quote>dense</> region of query words is
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contains a more <quote>dense</> region of query words is
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assigned a higher rank than one with scattered query words.
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</para>
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</listitem>
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@ -1151,13 +1151,13 @@ MaxFragments=0, FragmentDelimiter=" ... "
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<screen>
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SELECT ts_headline('english',
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'The most common type of search
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is to find all documents containing given query terms
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is to find all documents containing given query terms
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and return them in order of their similarity to the
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query.',
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to_tsquery('query & similarity'));
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ts_headline
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------------------------------------------------------------
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containing given <b>query</b> terms
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containing given <b>query</b> terms
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and return them in order of their <b>similarity</b> to the
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<b>query</b>.
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@ -1166,7 +1166,7 @@ SELECT ts_headline('english',
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is to find all documents containing given query terms
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and return them in order of their similarity to the
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query.',
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to_tsquery('query & similarity'),
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to_tsquery('query & similarity'),
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'StartSel = <, StopSel = >');
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ts_headline
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-------------------------------------------------------
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@ -2064,6 +2064,14 @@ SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.h
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(notice that one token can produce more than one lexeme)
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</para>
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</listitem>
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<listitem>
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<para>
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a single lexeme with the <literal>TSL_FILTER</> flag set, to replace
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the original token with a new token to be passed to subsequent
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dictionaries (a dictionary that does this is called a
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<firstterm>filtering dictionary</>)
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</para>
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</listitem>
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<listitem>
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<para>
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an empty array if the dictionary knows the token, but it is a stop word
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@ -2096,6 +2104,13 @@ SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.h
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until some dictionary recognizes it as a known word. If it is identified
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as a stop word, or if no dictionary recognizes the token, it will be
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discarded and not indexed or searched for.
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Normally, the first dictionary that returns a non-<literal>NULL</>
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output determines the result, and any remaining dictionaries are not
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consulted; but a filtering dictionary can replace the given word
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with a modified word, which is then passed to subsequent dictionaries.
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</para>
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<para>
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The general rule for configuring a list of dictionaries
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is to place first the most narrow, most specific dictionary, then the more
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general dictionaries, finishing with a very general dictionary, like
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@ -2112,6 +2127,16 @@ ALTER TEXT SEARCH CONFIGURATION astro_en
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</programlisting>
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</para>
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<para>
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A filtering dictionary can be placed anywhere in the list, except at the
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end where it'd be useless. Filtering dictionaries are useful to partially
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normalize words to simplify the task of later dictionaries. For example,
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a filtering dictionary could be used to remove accents from accented
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letters, as is done by the
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<link linkend="unaccent"><filename>contrib/unaccent</></link>
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extension module.
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</para>
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<sect2 id="textsearch-stopwords">
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<title>Stop Words</title>
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@ -2184,7 +2209,7 @@ CREATE TEXT SEARCH DICTIONARY public.simple_dict (
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Here, <literal>english</literal> is the base name of a file of stop words.
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The file's full name will be
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<filename>$SHAREDIR/tsearch_data/english.stop</>,
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where <literal>$SHAREDIR</> means the
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where <literal>$SHAREDIR</> means the
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<productname>PostgreSQL</productname> installation's shared-data directory,
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often <filename>/usr/local/share/postgresql</> (use <command>pg_config
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--sharedir</> to determine it if you're not sure).
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@ -2295,63 +2320,6 @@ SELECT * FROM ts_debug('english', 'Paris');
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asciiword | Word, all ASCII | Paris | {my_synonym,english_stem} | my_synonym | {paris}
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</screen>
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</para>
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<para>
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An asterisk (<literal>*</literal>) at the end of definition word indicates
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that definition word is a prefix, and <function>to_tsquery()</function>
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function will transform that definition to the prefix search format (see
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<xref linkend="textsearch-parsing-queries">).
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Notice that it is ignored in <function>to_tsvector()</function>.
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</para>
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<para>
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Contents of <filename>$SHAREDIR/tsearch_data/synonym_sample.syn</>:
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<programlisting>
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postgres pgsql
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postgresql pgsql
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postgre pgsql
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gogle googl
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indices index*
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</programlisting>
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</para>
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<para>
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Results:
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<screen>
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=# CREATE TEXT SEARCH DICTIONARY syn (template=synonym, synonyms='synonym_sample');
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=# SELECT ts_lexize('syn','indices');
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ts_lexize
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-----------
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{index}
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(1 row)
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=# CREATE TEXT SEARCH CONFIGURATION tst (copy=simple);
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=# ALTER TEXT SEARCH CONFIGURATION tst ALTER MAPPING FOR asciiword WITH syn;
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=# SELECT to_tsquery('tst','indices');
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to_tsquery
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------------
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'index':*
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(1 row)
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=# SELECT 'indexes are very useful'::tsvector;
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tsvector
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---------------------------------
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'are' 'indexes' 'useful' 'very'
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(1 row)
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=# SELECT 'indexes are very useful'::tsvector @@ to_tsquery('tst','indices');
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?column?
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----------
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t
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(1 row)
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=# SELECT to_tsvector('tst','indices');
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to_tsvector
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-------------
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'index':1
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(1 row)
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</screen>
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</para>
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<para>
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The only parameter required by the <literal>synonym</> template is
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@ -2374,6 +2342,60 @@ indices index*
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<literal>true</>, words and tokens are not folded to lower case,
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but are compared as-is.
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</para>
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<para>
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An asterisk (<literal>*</literal>) can be placed at the end of a synonym
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in the configuration file. This indicates that the synonym is a prefix.
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The asterisk is ignored when the entry is used in
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<function>to_tsvector()</function>, but when it is used in
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<function>to_tsquery()</function>, the result will be a query item with
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the prefix match marker (see
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<xref linkend="textsearch-parsing-queries">).
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For example, suppose we have these entries in
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<filename>$SHAREDIR/tsearch_data/synonym_sample.syn</>:
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<programlisting>
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postgres pgsql
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postgresql pgsql
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postgre pgsql
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gogle googl
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indices index*
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</programlisting>
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Then we will get these results:
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<screen>
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mydb=# CREATE TEXT SEARCH DICTIONARY syn (template=synonym, synonyms='synonym_sample');
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mydb=# SELECT ts_lexize('syn','indices');
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ts_lexize
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-----------
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{index}
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(1 row)
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mydb=# CREATE TEXT SEARCH CONFIGURATION tst (copy=simple);
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mydb=# ALTER TEXT SEARCH CONFIGURATION tst ALTER MAPPING FOR asciiword WITH syn;
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mydb=# SELECT to_tsvector('tst','indices');
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to_tsvector
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-------------
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'index':1
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(1 row)
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mydb=# SELECT to_tsquery('tst','indices');
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to_tsquery
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------------
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'index':*
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(1 row)
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mydb=# SELECT 'indexes are very useful'::tsvector;
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tsvector
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---------------------------------
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'are' 'indexes' 'useful' 'very'
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(1 row)
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mydb=# SELECT 'indexes are very useful'::tsvector @@ to_tsquery('tst','indices');
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?column?
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----------
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t
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(1 row)
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</screen>
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</para>
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</sect2>
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<sect2 id="textsearch-thesaurus">
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@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/unaccent.sgml,v 1.6 2010/08/25 02:12:00 tgl Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/unaccent.sgml,v 1.7 2010/08/25 21:42:55 tgl Exp $ -->
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<sect1 id="unaccent">
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<title>unaccent</title>
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@ -75,8 +75,10 @@
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<para>
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Running the installation script <filename>unaccent.sql</> creates a text
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search template <literal>unaccent</> and a dictionary <literal>unaccent</>
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based on it, with default parameters. You can alter the
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parameters, for example
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based on it. The <literal>unaccent</> dictionary has the default
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parameter setting <literal>RULES='unaccent'</>, which makes it immediately
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usable with the standard <filename>unaccent.rules</> file.
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If you wish, you can alter the parameter, for example
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<programlisting>
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mydb=# ALTER TEXT SEARCH DICTIONARY unaccent (RULES='my_rules');
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