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.

doc/src/sgml/textsearch.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.58 2010/08/20 13:59:45 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.59 2010/08/25 21:42:55 tgl Exp $ -->
 
 <chapter id="textsearch">
  <title>Full Text Search</title>
@@ -112,7 +112,7 @@
      as a sorted array of normalized lexemes. Along with the lexemes it is
      often desirable to store positional information to use for
      <firstterm>proximity ranking</firstterm>, so that a document that
-     contains a more <quote>dense</> region of query words is 
+     contains a more <quote>dense</> region of query words is
      assigned a higher rank than one with scattered query words.
     </para>
    </listitem>
@@ -1151,13 +1151,13 @@ MaxFragments=0, FragmentDelimiter=" ... "
 <screen>
 SELECT ts_headline('english',
   'The most common type of search
-is to find all documents containing given query terms 
+is to find all documents containing given query terms
 and return them in order of their similarity to the
 query.',
   to_tsquery('query &amp; similarity'));
                         ts_headline                         
 ------------------------------------------------------------
- containing given &lt;b&gt;query&lt;/b&gt; terms 
+ containing given &lt;b&gt;query&lt;/b&gt; terms
  and return them in order of their &lt;b&gt;similarity&lt;/b&gt; to the
  &lt;b&gt;query&lt;/b&gt;.
 
@@ -1166,7 +1166,7 @@ SELECT ts_headline('english',
 is to find all documents containing given query terms
 and return them in order of their similarity to the
 query.',
-  to_tsquery('query &amp; similarity'), 
+  to_tsquery('query &amp; similarity'),
   'StartSel = &lt;, StopSel = &gt;');
                       ts_headline                      
 -------------------------------------------------------
@@ -2064,6 +2064,14 @@ SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.h
       (notice that one token can produce more than one lexeme)
      </para>
     </listitem>
+    <listitem>
+     <para>
+      a single lexeme with the <literal>TSL_FILTER</> flag set, to replace
+      the original token with a new token to be passed to subsequent
+      dictionaries (a dictionary that does this is called a
+      <firstterm>filtering dictionary</>)
+     </para>
+    </listitem>
     <listitem>
      <para>
       an empty array if the dictionary knows the token, but it is a stop word
@@ -2096,6 +2104,13 @@ SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.h
    until some dictionary recognizes it as a known word.  If it is identified
    as a stop word, or if no dictionary recognizes the token, it will be
    discarded and not indexed or searched for.
+   Normally, the first dictionary that returns a non-<literal>NULL</>
+   output determines the result, and any remaining dictionaries are not
+   consulted; but a filtering dictionary can replace the given word
+   with a modified word, which is then passed to subsequent dictionaries.
+  </para>
+
+  <para>
    The general rule for configuring a list of dictionaries
    is to place first the most narrow, most specific dictionary, then the more
    general dictionaries, finishing with a very general dictionary, like
@@ -2112,6 +2127,16 @@ ALTER TEXT SEARCH CONFIGURATION astro_en
 </programlisting>
   </para>
 
+  <para>
+   A filtering dictionary can be placed anywhere in the list, except at the
+   end where it'd be useless.  Filtering dictionaries are useful to partially
+   normalize words to simplify the task of later dictionaries.  For example,
+   a filtering dictionary could be used to remove accents from accented
+   letters, as is done by the
+   <link linkend="unaccent"><filename>contrib/unaccent</></link>
+   extension module.
+  </para>
+
   <sect2 id="textsearch-stopwords">
    <title>Stop Words</title>
 
@@ -2184,7 +2209,7 @@ CREATE TEXT SEARCH DICTIONARY public.simple_dict (
     Here, <literal>english</literal> is the base name of a file of stop words.
     The file's full name will be
     <filename>$SHAREDIR/tsearch_data/english.stop</>,
-    where <literal>$SHAREDIR</> means the 
+    where <literal>$SHAREDIR</> means the
     <productname>PostgreSQL</productname> installation's shared-data directory,
     often <filename>/usr/local/share/postgresql</> (use <command>pg_config
     --sharedir</> to determine it if you're not sure).
@@ -2295,17 +2320,39 @@ SELECT * FROM ts_debug('english', 'Paris');
  asciiword | Word, all ASCII | Paris | {my_synonym,english_stem} | my_synonym | {paris}
 </screen>
    </para>
-   
+
    <para>
-    An asterisk (<literal>*</literal>) at the end of definition word indicates 
-    that definition word is a prefix, and <function>to_tsquery()</function> 
-    function will transform that definition to the prefix search format (see 
-    <xref linkend="textsearch-parsing-queries">). 
-    Notice that it is ignored in <function>to_tsvector()</function>.
+    The only parameter required by the <literal>synonym</> template is
+    <literal>SYNONYMS</>, which is the base name of its configuration file
+    &mdash; <literal>my_synonyms</> in the above example.
+    The file's full name will be
+    <filename>$SHAREDIR/tsearch_data/my_synonyms.syn</>
+    (where <literal>$SHAREDIR</> means the
+    <productname>PostgreSQL</> installation's shared-data directory).
+    The file format is just one line
+    per word to be substituted, with the word followed by its synonym,
+    separated by white space.  Blank lines and trailing spaces are ignored.
+   </para>
+
+   <para>
+    The <literal>synonym</> template also has an optional parameter
+    <literal>CaseSensitive</>, which defaults to <literal>false</>.  When
+    <literal>CaseSensitive</> is <literal>false</>, words in the synonym file
+    are folded to lower case, as are input tokens.  When it is
+    <literal>true</>, words and tokens are not folded to lower case,
+    but are compared as-is.
    </para>
 
    <para>
-    Contents of <filename>$SHAREDIR/tsearch_data/synonym_sample.syn</>:
+    An asterisk (<literal>*</literal>) can be placed at the end of a synonym
+    in the configuration file.  This indicates that the synonym is a prefix.
+    The asterisk is ignored when the entry is used in
+    <function>to_tsvector()</function>, but when it is used in
+    <function>to_tsquery()</function>, the result will be a query item with
+    the prefix match marker (see
+    <xref linkend="textsearch-parsing-queries">).
+    For example, suppose we have these entries in
+    <filename>$SHAREDIR/tsearch_data/synonym_sample.syn</>:
 <programlisting>
 postgres        pgsql
 postgresql      pgsql
@@ -2313,67 +2360,42 @@ postgre pgsql
 gogle   googl
 indices index*
 </programlisting>
-   </para>
-
-   <para>
-    Results:
+    Then we will get these results:
 <screen>
-=# CREATE TEXT SEARCH DICTIONARY syn (template=synonym, synonyms='synonym_sample');
-=# SELECT ts_lexize('syn','indices');
+mydb=# CREATE TEXT SEARCH DICTIONARY syn (template=synonym, synonyms='synonym_sample');
+mydb=# SELECT ts_lexize('syn','indices');
  ts_lexize
 -----------
  {index}
 (1 row)
 
-=# CREATE TEXT SEARCH CONFIGURATION tst (copy=simple);
-=# ALTER TEXT SEARCH CONFIGURATION tst ALTER MAPPING FOR asciiword WITH syn;
-=# SELECT to_tsquery('tst','indices');
+mydb=# CREATE TEXT SEARCH CONFIGURATION tst (copy=simple);
+mydb=# ALTER TEXT SEARCH CONFIGURATION tst ALTER MAPPING FOR asciiword WITH syn;
+mydb=# SELECT to_tsvector('tst','indices');
+ to_tsvector
+-------------
+ 'index':1
+(1 row)
+
+mydb=# SELECT to_tsquery('tst','indices');
  to_tsquery
 ------------
  'index':*
 (1 row)
 
-=# SELECT 'indexes are very useful'::tsvector;
+mydb=# SELECT 'indexes are very useful'::tsvector;
             tsvector             
 ---------------------------------
  'are' 'indexes' 'useful' 'very'
 (1 row)
 
-=# SELECT 'indexes are very useful'::tsvector @@ to_tsquery('tst','indices');
+mydb=# SELECT 'indexes are very useful'::tsvector @@ to_tsquery('tst','indices');
  ?column?
 ----------
  t
 (1 row)
-
-=# SELECT to_tsvector('tst','indices');
- to_tsvector
--------------
- 'index':1
-(1 row)
 </screen>
    </para>
-
-   <para>
-    The only parameter required by the <literal>synonym</> template is
-    <literal>SYNONYMS</>, which is the base name of its configuration file
-    &mdash; <literal>my_synonyms</> in the above example.
-    The file's full name will be
-    <filename>$SHAREDIR/tsearch_data/my_synonyms.syn</>
-    (where <literal>$SHAREDIR</> means the
-    <productname>PostgreSQL</> installation's shared-data directory).
-    The file format is just one line
-    per word to be substituted, with the word followed by its synonym,
-    separated by white space.  Blank lines and trailing spaces are ignored.
-   </para>
-
-   <para>
-    The <literal>synonym</> template also has an optional parameter
-    <literal>CaseSensitive</>, which defaults to <literal>false</>.  When
-    <literal>CaseSensitive</> is <literal>false</>, words in the synonym file
-    are folded to lower case, as are input tokens.  When it is
-    <literal>true</>, words and tokens are not folded to lower case,
-    but are compared as-is.
-   </para>
   </sect2>
 
   <sect2 id="textsearch-thesaurus">

doc/src/sgml/unaccent.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/unaccent.sgml,v 1.6 2010/08/25 02:12:00 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/unaccent.sgml,v 1.7 2010/08/25 21:42:55 tgl Exp $ -->
 
 <sect1 id="unaccent">
  <title>unaccent</title>
@@ -75,8 +75,10 @@
   <para>
    Running the installation script <filename>unaccent.sql</> creates a text
    search template <literal>unaccent</> and a dictionary <literal>unaccent</>
-   based on it, with default parameters.  You can alter the
-   parameters, for example
+   based on it.  The <literal>unaccent</> dictionary has the default
+   parameter setting <literal>RULES='unaccent'</>, which makes it immediately
+   usable with the standard <filename>unaccent.rules</> file.
+   If you wish, you can alter the parameter, for example
 
 <programlisting>
 mydb=# ALTER TEXT SEARCH DICTIONARY unaccent (RULES='my_rules');