<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.40 2007/12/13 06:32:47 tgl Exp $ -->

<chapter id="textsearch">
 <title id="textsearch-title">Full Text Search</title>

  <indexterm zone="textsearch">
   <primary>full text search</primary>
  </indexterm>

  <indexterm zone="textsearch">
   <primary>text search</primary>
  </indexterm>

 <sect1 id="textsearch-intro">
  <title>Introduction</title>

  <para>
   Full Text Searching (or just <firstterm>text search</firstterm>) provides
   the capability to identify natural-language <firstterm>documents</> that
   satisfy a <firstterm>query</firstterm>, and optionally to sort them by
   relevance to the query.  The most common type of search
   is to find all documents containing given <firstterm>query terms</firstterm>
   and return them in order of their <firstterm>similarity</firstterm> to the
   query.  Notions of <varname>query</varname> and
   <varname>similarity</varname> are very flexible and depend on the specific
   application. The simplest search considers <varname>query</varname> as a
   set of words and <varname>similarity</varname> as the frequency of query
   words in the document.
  </para>

  <para>
   Textual search operators have existed in databases for years.
   <productname>PostgreSQL</productname> has
   <literal>~</literal>, <literal>~*</literal>, <literal>LIKE</literal>, and
   <literal>ILIKE</literal> operators for textual data types, but they lack
   many essential properties required by modern information systems:
  </para>

  <itemizedlist  spacing="compact" mark="bullet">
   <listitem>
    <para>
     There is no linguistic support, even for English.  Regular expressions
     are not sufficient because they cannot easily handle derived words, e.g.,
     <literal>satisfies</literal> and <literal>satisfy</literal>. You might
     miss documents that contain <literal>satisfies</literal>, although you
     probably would like to find them when searching for
     <literal>satisfy</literal>. It is possible to use <literal>OR</literal>
     to search for multiple derived forms, but this is tedious and error-prone
     (some words can have several thousand derivatives).
    </para>
   </listitem>

   <listitem>
    <para>
     They provide no ordering (ranking) of search results, which makes them
     ineffective when thousands of matching documents are found.
    </para>
   </listitem>

   <listitem>
    <para>
     They tend to be slow because there is no index support, so they must
     process all documents for every search.
    </para>
   </listitem>
  </itemizedlist>

  <para>
   Full text indexing allows documents to be <emphasis>preprocessed</emphasis>
   and an index saved for later rapid searching. Preprocessing includes:
  </para>

  <itemizedlist  mark="none">
   <listitem>
    <para>
     <emphasis>Parsing documents into <firstterm>tokens</></emphasis>. It is
     useful to identify various classes of tokens, e.g. numbers, words,
     complex words, email addresses, so that they can be processed
     differently.  In principle token classes depend on the specific
     application, but for most purposes it is adequate to use a predefined
     set of classes.
     <productname>PostgreSQL</productname> uses a <firstterm>parser</> to
     perform this step.  A standard parser is provided, and custom parsers
     can be created for specific needs.
    </para>
   </listitem>

   <listitem>
    <para>
     <emphasis>Converting tokens into <firstterm>lexemes</></emphasis>.
     A lexeme is a string, just like a token, but it has been
     <firstterm>normalized</> so that different forms of the same word
     are made alike.  For example, normalization almost always includes
     folding upper-case letters to lower-case, and often involves removal
     of suffixes (such as <literal>s</> or <literal>es</> in English).
     This allows searches to find variant forms of the
     same word, without tediously entering all the possible variants.
     Also, this step typically eliminates <firstterm>stop words</>, which
     are words that are so common that they are useless for searching.
     (In short, then, tokens are raw fragments of the document text, while
     lexemes are words that are believed useful for indexing and searching.)
     <productname>PostgreSQL</productname> uses <firstterm>dictionaries</> to
     perform this step.  Various standard dictionaries are provided, and
     custom ones can be created for specific needs.
    </para>
   </listitem>

   <listitem>
    <para>
     <emphasis>Storing preprocessed documents optimized for
     searching</emphasis>.  For example, each document can be represented
     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 
     assigned a higher rank than one with scattered query words.
    </para>
   </listitem>
  </itemizedlist>

  <para>
   Dictionaries allow fine-grained control over how tokens are normalized.
   With appropriate dictionaries, you can:
  </para>

  <itemizedlist  spacing="compact" mark="bullet">
   <listitem>
    <para>
     Define stop words that should not be indexed.
    </para>
   </listitem>

   <listitem>
    <para>
     Map synonyms to a single word using <application>Ispell</>.
    </para>
   </listitem>

   <listitem>
    <para>
     Map phrases to a single word using a thesaurus.
    </para>
   </listitem>

   <listitem>
    <para>
     Map different variations of a word to a canonical form using
     an <application>Ispell</> dictionary.
    </para>
   </listitem>

   <listitem>
    <para>
     Map different variations of a word to a canonical form using
     <application>Snowball</> stemmer rules.
    </para>
   </listitem>
  </itemizedlist>

  <para>
   A data type <type>tsvector</type> is provided for storing preprocessed
   documents, along with a type <type>tsquery</type> for representing processed
   queries (<xref linkend="datatype-textsearch">).  There are many
   functions and operators available for these data types
   (<xref linkend="functions-textsearch">), the most important of which is
   the match operator <literal>@@</literal>, which we introduce in
   <xref linkend="textsearch-matching">.  Full text searches can be accelerated
   using indexes (<xref linkend="textsearch-indexes">).
  </para>


  <sect2 id="textsearch-document">
   <title>What Is a Document?</title>

   <indexterm zone="textsearch-document">
    <primary>document</primary>
    <secondary>text search</secondary>
   </indexterm>

   <para>
    A <firstterm>document</> is the unit of searching in a full text search
    system; for example, a magazine article or email message.  The text search
    engine must be able to parse documents and store associations of lexemes
    (key words) with their parent document. Later, these associations are
    used to search for documents that contain query words.
   </para>

   <para>
    For searches within <productname>PostgreSQL</productname>,
    a document is normally a textual field within a row of a database table,
    or possibly a combination (concatenation) of such fields, perhaps stored
    in several tables or obtained dynamically. In other words, a document can
    be constructed from different parts for indexing and it might not be
    stored anywhere as a whole. For example:

<programlisting>
SELECT title || ' ' ||  author || ' ' ||  abstract || ' ' || body AS document
FROM messages
WHERE mid = 12;

SELECT m.title || ' ' || m.author || ' ' || m.abstract || ' ' || d.body AS document
FROM messages m, docs d
WHERE mid = did AND mid = 12;
</programlisting>
   </para>

   <note>
    <para>
     Actually, in these example queries, <function>coalesce</function>
     should be used to prevent a single <literal>NULL</literal> attribute from
     causing a <literal>NULL</literal> result for the whole document.
    </para>
   </note>

   <para>
    Another possibility is to store the documents as simple text files in the
    file system. In this case, the database can be used to store the full text
    index and to execute searches, and some unique identifier can be used to
    retrieve the document from the file system.  However, retrieving files
    from outside the database requires superuser permissions or special
    function support, so this is usually less convenient than keeping all
    the data inside <productname>PostgreSQL</productname>.  Also, keeping
    everything inside the database allows easy access
    to document metadata to assist in indexing and display.
   </para>

   <para>
    For text search purposes, each document must be reduced to the
    preprocessed <type>tsvector</> format.  Searching and ranking
    are performed entirely on the <type>tsvector</> representation
    of a document &mdash; the original text need only be retrieved
    when the document has been selected for display to a user.
    We therefore often speak of the <type>tsvector</> as being the
    document, but of course it is only a compact representation of
    the full document.
   </para>
  </sect2>

  <sect2 id="textsearch-matching">
   <title>Basic Text Matching</title>

   <para>
    Full text searching in <productname>PostgreSQL</productname> is based on
    the match operator <literal>@@</literal>, which returns
    <literal>true</literal> if a <type>tsvector</type>
    (document) matches a <type>tsquery</type> (query).
    It doesn't matter which data type is written first:

<programlisting>
SELECT 'a fat cat sat on a mat and ate a fat rat'::tsvector @@ 'cat &amp; rat'::tsquery;
 ?column?
----------
 t

SELECT 'fat &amp; cow'::tsquery @@ 'a fat cat sat on a mat and ate a fat rat'::tsvector;
 ?column?
----------
 f
</programlisting>
   </para>

   <para>
    As the above example suggests, a <type>tsquery</type> is not just raw
    text, any more than a <type>tsvector</type> is.  A <type>tsquery</type>
    contains search terms, which must be already-normalized lexemes, and
    may combine multiple terms using AND, OR, and NOT operators.
    (For details see <xref linkend="datatype-textsearch">.)  There are
    functions <function>to_tsquery</> and <function>plainto_tsquery</>
    that are helpful in converting user-written text into a proper
    <type>tsquery</type>, for example by normalizing words appearing in
    the text.  Similarly, <function>to_tsvector</> is used to parse and
    normalize a document string.  So in practice a text search match would
    look more like this:

<programlisting>
SELECT to_tsvector('fat cats ate fat rats') @@ to_tsquery('fat &amp; rat');
 ?column? 
----------
 t
</programlisting>

    Observe that this match would not succeed if written as

<programlisting>
SELECT 'fat cats ate fat rats'::tsvector @@ to_tsquery('fat &amp; rat');
 ?column? 
----------
 f
</programlisting>

    since here no normalization of the word <literal>rats</> will occur.
    The elements of a <type>tsvector</> are lexemes, which are assumed
    already normalized, so <literal>rats</> does not match <literal>rat</>.
   </para>

   <para>
    The <literal>@@</literal> operator also
    supports <type>text</type> input, allowing explicit conversion of a text
    string to <type>tsvector</type> or <type>tsquery</> to be skipped
    in simple cases.  The variants available are:

<programlisting>
tsvector @@ tsquery
tsquery  @@ tsvector
text @@ tsquery
text @@ text
</programlisting>
   </para>

   <para>
    The first two of these we saw already.
    The form <type>text</type> <literal>@@</literal> <type>tsquery</type>
    is equivalent to <literal>to_tsvector(x) @@ y</literal>.
    The form <type>text</type> <literal>@@</literal> <type>text</type>
    is equivalent to <literal>to_tsvector(x) @@ plainto_tsquery(y)</literal>.
   </para>
  </sect2>

  <sect2 id="textsearch-intro-configurations">
   <title>Configurations</title>

   <para>
    The above are all simple text search examples.  As mentioned before, full
    text search functionality includes the ability to do many more things:
    skip indexing certain words (stop words), process synonyms, and use
    sophisticated parsing, e.g. parse based on more than just white space.
    This functionality is controlled by <firstterm>text search
    configurations</>.  <productname>PostgreSQL</> comes with predefined
    configurations for many languages, and you can easily create your own
    configurations.  (<application>psql</>'s <command>\dF</> command
    shows all available configurations.)
   </para>

   <para>
    During installation an appropriate configuration is selected and
    <xref linkend="guc-default-text-search-config"> is set accordingly
    in <filename>postgresql.conf</>.  If you are using the same text search
    configuration for the entire cluster you can use the value in
    <filename>postgresql.conf</>.  To use different configurations
    throughout the cluster but the same configuration within any one database,
    use <command>ALTER DATABASE ... SET</>.  Otherwise, you can set
    <varname>default_text_search_config</varname> in each session.
   </para>

   <para>
    Each text search function that depends on a configuration has an optional
    <type>regconfig</> argument, so that the configuration to use can be
    specified explicitly.  <varname>default_text_search_config</varname>
    is used only when this argument is omitted.
   </para>

   <para>
    To make it easier to build custom text search configurations, a
    configuration is built up from simpler database objects.
    <productname>PostgreSQL</>'s text search facility provides
    four types of configuration-related database objects:
   </para>

  <itemizedlist  spacing="compact" mark="bullet">
   <listitem>
    <para>
     <firstterm>Text search parsers</> break documents into tokens
     and classify each token (for example, as words or numbers).
    </para>
   </listitem>

   <listitem>
    <para>
     <firstterm>Text search dictionaries</> convert tokens to normalized
     form and reject stop words.
    </para>
   </listitem>

   <listitem>
    <para>
     <firstterm>Text search templates</> provide the functions underlying
     dictionaries.  (A dictionary simply specifies a template and a set
     of parameters for the template.)
    </para>
   </listitem>

   <listitem>
    <para>
     <firstterm>Text search configurations</> select a parser and a set
     of dictionaries to use to normalize the tokens produced by the parser.
    </para>
   </listitem>
  </itemizedlist>

   <para>
    Text search parsers and templates are built from low-level C functions;
    therefore it requires C programming ability to develop new ones, and
    superuser privileges to install one into a database.  (There are examples
    of add-on parsers and templates in the <filename>contrib/</> area of the
    <productname>PostgreSQL</> distribution.)  Since dictionaries and
    configurations just parameterize and connect together some underlying
    parsers and templates, no special privilege is needed to create a new
    dictionary or configuration.  Examples of creating custom dictionaries and
    configurations appear later in this chapter.
   </para>

  </sect2>

 </sect1>

 <sect1 id="textsearch-tables">
  <title>Tables and Indexes</title>

  <para>
   The examples in the previous section illustrated full text matching using
   simple constant strings.  This section shows how to search table data,
   optionally using indexes.
  </para>

  <sect2 id="textsearch-tables-search">
   <title>Searching a Table</title>

   <para>
    It is possible to do full text search with no index.  A simple query
    to print the <structname>title</> of each row that contains the word
    <literal>friend</> in its <structfield>body</> field is:

<programlisting>
SELECT title
FROM pgweb
WHERE to_tsvector('english', body) @@ to_tsquery('english', 'friend');
</programlisting>

    This will also find related words such as <literal>friends</>
    and <literal>friendly</>, since all these are reduced to the same
    normalized lexeme.
   </para>

   <para>
    The query above specifies that the <literal>english</> configuration
    is to be used to parse and normalize the strings.  Alternatively we
    could omit the configuration parameters:

<programlisting>
SELECT title
FROM pgweb
WHERE to_tsvector(body) @@ to_tsquery('friend');
</programlisting>

    This query will use the configuration set by <xref
    linkend="guc-default-text-search-config">.
   </para>

   <para>
    A more complex example is to
    select the ten most recent documents that contain <literal>create</> and
    <literal>table</> in the <structname>title</> or <structname>body</>:

<programlisting>
SELECT title
FROM pgweb
WHERE to_tsvector(title || body) @@ to_tsquery('create &amp; table')
ORDER BY last_mod_date DESC LIMIT 10;
</programlisting>

    For clarity we omitted the <function>coalesce</function> function
    which would be needed to search rows that contain <literal>NULL</literal>
    in one of the two fields.
   </para>

   <para>
    Although these queries will work without an index, most applications
    will find this approach too slow, except perhaps for occasional ad-hoc
    searches.  Practical use of text searching usually requires creating
    an index.
   </para>

  </sect2>

  <sect2 id="textsearch-tables-index">
   <title>Creating Indexes</title>

   <para>
    We can create a <acronym>GIN</acronym> index (<xref
    linkend="textsearch-indexes">) to speed up text searches:

<programlisting>
CREATE INDEX pgweb_idx ON pgweb USING gin(to_tsvector('english', body));
</programlisting>

    Notice that the 2-argument version of <function>to_tsvector</function> is
    used.  Only text search functions that specify a configuration name can
    be used in expression indexes (<xref linkend="indexes-expressional">).
    This is because the index contents must be unaffected by <xref
    linkend="guc-default-text-search-config">.  If they were affected, the
    index contents might be inconsistent because different entries could
    contain <type>tsvector</>s that were created with different text search
    configurations, and there would be no way to guess which was which.  It
    would be impossible to dump and restore such an index correctly.
   </para>

   <para>
    Because the two-argument version of <function>to_tsvector</function> was
    used in the index above, only a query reference that uses the 2-argument
    version of <function>to_tsvector</function> with the same configuration
    name will use that index.  That is, <literal>WHERE
    to_tsvector('english', body) @@ 'a &amp; b'</> can use the index,
    but <literal>WHERE to_tsvector(body) @@ 'a &amp; b'</> cannot.
    This ensures that an index will be used only with the same configuration
    used to create the index entries.
   </para>

  <para>
    It is possible to set up more complex expression indexes wherein the
    configuration name is specified by another column, e.g.:

<programlisting>
CREATE INDEX pgweb_idx ON pgweb USING gin(to_tsvector(config_name, body));
</programlisting>

    where <literal>config_name</> is a column in the <literal>pgweb</>
    table.  This allows mixed configurations in the same index while
    recording which configuration was used for each index entry.  This
    would be useful, for example, if the document collection contained
    documents in different languages.  Again,
    queries that are to use the index must be phrased to match, e.g.
    <literal>WHERE to_tsvector(config_name, body) @@ 'a &amp; b'</>.
   </para>

   <para>
    Indexes can even concatenate columns:

<programlisting>
CREATE INDEX pgweb_idx ON pgweb USING gin(to_tsvector('english', title || body));
</programlisting>
   </para>

   <para>
    Another approach is to create a separate <type>tsvector</> column
    to hold the output of <function>to_tsvector</>.  This example is a
    concatenation of <literal>title</literal> and <literal>body</literal>,
    using <function>coalesce</> to ensure that one field will still be
    indexed when the other is <literal>NULL</>:

<programlisting>
ALTER TABLE pgweb ADD COLUMN textsearchable_index_col tsvector;
UPDATE pgweb SET textsearchable_index_col =
     to_tsvector('english', coalesce(title,'') || coalesce(body,''));
</programlisting>

    Then we create a <acronym>GIN</acronym> index to speed up the search:

<programlisting>
CREATE INDEX textsearch_idx ON pgweb USING gin(textsearchable_index_col);
</programlisting>

    Now we are ready to perform a fast full text search:

<programlisting>
SELECT title
FROM pgweb
WHERE textsearchable_index_col @@ to_tsquery('create &amp; table')
ORDER BY last_mod_date DESC LIMIT 10;
</programlisting>
   </para>

   <para>
    When using a separate column to store the <type>tsvector</>
    representation,
    it is necessary to create a trigger to keep the <type>tsvector</>
    column current anytime <literal>title</> or <literal>body</> changes.
    <xref linkend="textsearch-update-triggers"> explains how to do that.
   </para>

   <para>
    One advantage of the separate-column approach over an expression index
    is that it is not necessary to explicitly specify the text search
    configuration in queries in order to make use of the index.  As shown
    in the example above, the query can depend on
    <varname>default_text_search_config</>.  Another advantage is that
    searches will be faster, since it will not be necessary to redo the
    <function>to_tsvector</> calls to verify index matches.  (This is more
    important when using a GiST index than a GIN index; see <xref
    linkend="textsearch-indexes">.)  The expression-index approach is
    simpler to set up, however, and it requires less disk space since the
    <type>tsvector</> representation is not stored explicitly.
   </para>

  </sect2>

 </sect1>

 <sect1 id="textsearch-controls">
  <title>Controlling Text Search</title>

  <para>
   To implement full text searching there must be a function to create a
   <type>tsvector</type> from a document and a <type>tsquery</type> from a
   user query. Also, we need to return results in a useful order, so we need
   a function that compares documents with respect to their relevance to
   the query. It's also important to be able to display the results nicely.
   <productname>PostgreSQL</productname> provides support for all of these
   functions.
  </para>

  <sect2 id="textsearch-parsing-documents">
   <title>Parsing Documents</title>

   <para>
    <productname>PostgreSQL</productname> provides the
    function <function>to_tsvector</function> for converting a document to
    the <type>tsvector</type> data type.
   </para>

   <indexterm>
    <primary>to_tsvector</primary>
   </indexterm>

   <synopsis>
    to_tsvector(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">document</replaceable> <type>text</>) returns <type>tsvector</>
   </synopsis>

   <para>
    <function>to_tsvector</function> parses a textual document into tokens,
    reduces the tokens to lexemes, and returns a <type>tsvector</type> which
    lists the lexemes together with their positions in the document.
    The document is processed according to the specified or default
    text search configuration.
    Here is a simple example:

<programlisting>
SELECT to_tsvector('english', 'a fat  cat sat on a mat - it ate a fat rats');
                  to_tsvector
-----------------------------------------------------
 'ate':9 'cat':3 'fat':2,11 'mat':7 'rat':12 'sat':4
</programlisting>
   </para>

   <para>
    In the example above we see that the resulting <type>tsvector</type> does not
    contain the words <literal>a</literal>, <literal>on</literal>, or
    <literal>it</literal>, the word <literal>rats</literal> became
    <literal>rat</literal>, and the punctuation sign <literal>-</literal> was
    ignored.
   </para>

   <para>
    The <function>to_tsvector</function> function internally calls a parser
    which breaks the document text into tokens and assigns a type to
    each token.  For each token, a list of
    dictionaries (<xref linkend="textsearch-dictionaries">) is consulted,
    where the list can vary depending on the token type.  The first dictionary
    that <firstterm>recognizes</> the token emits one or more normalized
    <firstterm>lexemes</firstterm> to represent the token.  For example,
    <literal>rats</literal> became <literal>rat</literal> because one of the
    dictionaries recognized that the word <literal>rats</literal> is a plural
    form of <literal>rat</literal>.  Some words are recognized as
    <firstterm>stop words</> (<xref linkend="textsearch-stopwords">), which
    causes them to be ignored since they occur too frequently to be useful in
    searching.  In our example these are
    <literal>a</literal>, <literal>on</literal>, and <literal>it</literal>.
    If no dictionary in the list recognizes the token then it is also ignored.
    In this example that happened to the punctuation sign <literal>-</literal>
    because there are in fact no dictionaries assigned for its token type
    (<literal>Space symbols</literal>), meaning space tokens will never be
    indexed. The choices of parser, dictionaries and which types of tokens to
    index are determined by the selected text search configuration (<xref
    linkend="textsearch-configuration">).  It is possible to have
    many different configurations in the same database, and predefined
    configurations are available for various languages. In our example
    we used the default configuration <literal>english</literal> for the
    English language.
   </para>

   <para>
    The function <function>setweight</function> can be used to label the
    entries of a <type>tsvector</type> with a given <firstterm>weight</>,
    where a weight is one of the letters <literal>A</>, <literal>B</>,
    <literal>C</>, or <literal>D</>.
    This is typically used to mark entries coming from
    different parts of a document, such as title versus body.  Later, this
    information can be used for ranking of search results.
   </para>

   <para>
    Because <function>to_tsvector</function>(<literal>NULL</literal>) will
    return <literal>NULL</literal>, it is recommended to use
    <function>coalesce</function> whenever a field might be null.
    Here is the recommended method for creating
    a <type>tsvector</type> from a structured document:

<programlisting>
UPDATE tt SET ti =
    setweight(to_tsvector(coalesce(title,'')), 'A')    ||
    setweight(to_tsvector(coalesce(keyword,'')), 'B')  ||
    setweight(to_tsvector(coalesce(abstract,'')), 'C') ||
    setweight(to_tsvector(coalesce(body,'')), 'D');
</programlisting>

    Here we have used <function>setweight</function> to label the source
    of each lexeme in the finished <type>tsvector</type>, and then merged
    the labeled <type>tsvector</type> values using the <type>tsvector</>
    concatenation operator <literal>||</>.  (<xref
    linkend="textsearch-manipulate-tsvector"> gives details about these
    operations.)
   </para>

  </sect2>

  <sect2 id="textsearch-parsing-queries">
   <title>Parsing Queries</title>

   <para>
    <productname>PostgreSQL</productname> provides the
    functions <function>to_tsquery</function> and
    <function>plainto_tsquery</function> for converting a query to
    the <type>tsquery</type> data type.  <function>to_tsquery</function>
    offers access to more features than <function>plainto_tsquery</function>,
    but is less forgiving about its input.
   </para>

   <indexterm>
    <primary>to_tsquery</primary>
   </indexterm>

   <synopsis>
    to_tsquery(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">querytext</replaceable> <type>text</>) returns <type>tsquery</>
   </synopsis>

   <para>
    <function>to_tsquery</function> creates a <type>tsquery</> value from
    <replaceable>querytext</replaceable>, which must consist of single tokens
    separated by the Boolean operators <literal>&amp;</literal> (AND),
    <literal>|</literal> (OR) and <literal>!</literal> (NOT).  These operators
    can be grouped using parentheses.  In other words, the input to
    <function>to_tsquery</function> must already follow the general rules for
    <type>tsquery</> input, as described in <xref
    linkend="datatype-textsearch">.  The difference is that while basic
    <type>tsquery</> input takes the tokens at face value,
    <function>to_tsquery</function> normalizes each token to a lexeme using
    the specified or default configuration, and discards any tokens that are
    stop words according to the configuration.  For example:

<programlisting>
SELECT to_tsquery('english', 'The &amp; Fat &amp; Rats');
  to_tsquery   
---------------
 'fat' &amp; 'rat'
</programlisting>

    As in basic <type>tsquery</> input, weight(s) can be attached to each
    lexeme to restrict it to match only <type>tsvector</> lexemes of those
    weight(s).  For example:

<programlisting>
SELECT to_tsquery('english', 'Fat | Rats:AB');
    to_tsquery    
------------------
 'fat' | 'rat':AB
</programlisting>

    <function>to_tsquery</function> can also accept single-quoted
    phrases.  This is primarily useful when the configuration includes a
    thesaurus dictionary that may trigger on such phrases.
    In the example below, a thesaurus contains the rule <literal>supernovae
    stars : sn</literal>:

<programlisting>
SELECT to_tsquery('''supernovae stars'' &amp; !crab');
  to_tsquery
---------------
 'sn' &amp; !'crab'
</programlisting>

    Without quotes, <function>to_tsquery</function> will generate a syntax
    error for tokens that are not separated by an AND or OR operator.
   </para>

   <indexterm>
    <primary>plainto_tsquery</primary>
   </indexterm>

   <synopsis>
    plainto_tsquery(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">querytext</replaceable> <type>text</>) returns <type>tsquery</>
   </synopsis>

   <para>
    <function>plainto_tsquery</> transforms unformatted text
    <replaceable>querytext</replaceable> to <type>tsquery</type>.
    The text is parsed and normalized much as for <function>to_tsvector</>,
    then the <literal>&amp;</literal> (AND) Boolean operator is inserted
    between surviving words.
   </para>

   <para>
    Example:

<programlisting>
 SELECT plainto_tsquery('english', 'The Fat Rats');
 plainto_tsquery 
-----------------
 'fat' &amp; 'rat'
</programlisting>

    Note that <function>plainto_tsquery</> cannot
    recognize either Boolean operators or weight labels in its input:

<programlisting>
SELECT plainto_tsquery('english', 'The Fat &amp; Rats:C');
   plainto_tsquery   
---------------------
 'fat' &amp; 'rat' &amp; 'c'
</programlisting>

    Here, all the input punctuation was discarded as being space symbols.
   </para>

  </sect2>

  <sect2 id="textsearch-ranking">
   <title>Ranking Search Results</title>

   <para>
    Ranking attempts to measure how relevant documents are to a particular
    query, so that when there are many matches the most relevant ones can be
    shown first.  <productname>PostgreSQL</productname> provides two
    predefined ranking functions, which take into account lexical, proximity,
    and structural information; that is, they consider how often the query
    terms appear in the document, how close together the terms are in the
    document, and how important is the part of the document where they occur.
    However, the concept of relevancy is vague and very application-specific.
    Different applications might require additional information for ranking,
    e.g. document modification time.  The built-in ranking functions are only
    examples.  You can write your own ranking functions and/or combine their
    results with additional factors to fit your specific needs.
   </para>

   <para>
    The two ranking functions currently available are:

    <variablelist>

     <varlistentry>

      <indexterm>
       <primary>ts_rank</primary>
      </indexterm>

      <term>
       <synopsis>
        ts_rank(<optional> <replaceable class="PARAMETER">weights</replaceable> <type>float4[]</>, </optional> <replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>, <replaceable class="PARAMETER">query</replaceable> <type>tsquery</> <optional>, <replaceable class="PARAMETER">normalization</replaceable> <type>integer</> </optional>) returns <type>float4</>
       </synopsis>
      </term>

      <listitem>
       <para>
        Standard ranking function.<!-- TODO document this better -->
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>

      <indexterm>
       <primary>ts_rank_cd</primary>
      </indexterm>

      <term>
       <synopsis>
        ts_rank_cd(<optional> <replaceable class="PARAMETER">weights</replaceable> <type>float4[]</>, </optional> <replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>, <replaceable class="PARAMETER">query</replaceable> <type>tsquery</> <optional>, <replaceable class="PARAMETER">normalization</replaceable> <type>integer</> </optional>) returns <type>float4</>
       </synopsis>
      </term>

      <listitem>
       <para>
        This function computes the <firstterm>cover density</firstterm>
        ranking for the given document vector and query, as described in
        Clarke, Cormack, and Tudhope's "Relevance Ranking for One to Three
        Term Queries" in the journal "Information Processing and Management",
        1999.
       </para>

       <para>
        This function requires positional information in its input.
        Therefore it will not work on <quote>stripped</> <type>tsvector</>
        values &mdash; it will always return zero.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

   </para>

   <para>
    For both these functions,
    the optional <replaceable class="PARAMETER">weights</replaceable>
    argument offers the ability to weigh word instances more or less
    heavily depending on how they are labeled.  The weight arrays specify
    how heavily to weigh each category of word, in the order:

<programlisting>
{D-weight, C-weight, B-weight, A-weight}
</programlisting>

    If no <replaceable class="PARAMETER">weights</replaceable> are provided,
    then these defaults are used:

<programlisting>
{0.1, 0.2, 0.4, 1.0}
</programlisting>

    Typically weights are used to mark words from special areas of the
    document, like the title or an initial abstract, so that they can be
    treated as more or less important than words in the document body.
   </para>

   <para>
    Since a longer document has a greater chance of containing a query term
    it is reasonable to take into account document size, e.g. a hundred-word
    document with five instances of a search word is probably more relevant
    than a thousand-word document with five instances.  Both ranking functions
    take an integer <replaceable>normalization</replaceable> option that
    specifies whether and how a document's length should impact its rank.
    The integer option controls several behaviors, so it is a bit mask:
    you can specify one or more behaviors using
    <literal>|</literal> (for example, <literal>2|4</literal>).

    <itemizedlist  spacing="compact" mark="bullet">
     <listitem>
      <para>
       0 (the default) ignores the document length
      </para>
     </listitem>
     <listitem>
      <para>
       1 divides the rank by 1 + the logarithm of the document length
      </para>
     </listitem>
     <listitem>
      <para>
       2 divides the rank by the document length
      </para>
     </listitem>
     <listitem>
      <para>
       4 divides the rank by the mean harmonic distance between extents
       (this is implemented only by <function>ts_rank_cd</>)
      </para>
     </listitem>
     <listitem>
      <para>
       8 divides the rank by the number of unique words in document
      </para>
     </listitem>
     <listitem>
      <para>
       16 divides the rank by 1 + the logarithm of the number
       of unique words in document
      </para>
     </listitem>
     <listitem>
      <para>
       32 divides the rank by itself + 1
      </para>
     </listitem>
    </itemizedlist>

    If more than one flag bit is specified, the transformations are
    applied in the order listed.
   </para>

   <para>
    It is important to note that the ranking functions do not use any global
    information, so it is impossible to produce a fair normalization to 1% or
    100% as sometimes desired.  Normalization option 32
    (<literal>rank/(rank+1)</literal>) can be applied to scale all ranks
    into the range zero to one, but of course this is just a cosmetic change;
    it will not affect the ordering of the search results.
   </para>

   <para>
    Here is an example that selects only the ten highest-ranked matches:

<programlisting>
SELECT title, ts_rank_cd(textsearch, query) AS rank
FROM apod, to_tsquery('neutrino|(dark &amp; matter)') query
WHERE query @@ textsearch
ORDER BY rank DESC LIMIT 10;
                     title                     |   rank
-----------------------------------------------+----------
 Neutrinos in the Sun                          |      3.1
 The Sudbury Neutrino Detector                 |      2.4
 A MACHO View of Galactic Dark Matter          |  2.01317
 Hot Gas and Dark Matter                       |  1.91171
 The Virgo Cluster: Hot Plasma and Dark Matter |  1.90953
 Rafting for Solar Neutrinos                   |      1.9
 NGC 4650A: Strange Galaxy and Dark Matter     |  1.85774
 Hot Gas and Dark Matter                       |   1.6123
 Ice Fishing for Cosmic Neutrinos              |      1.6
 Weak Lensing Distorts the Universe            | 0.818218
</programlisting>

    This is the same example using normalized ranking:

<programlisting>
SELECT title, ts_rank_cd(textsearch, query, 32 /* rank/(rank+1) */ ) AS rank
FROM apod, to_tsquery('neutrino|(dark &amp; matter)') query
WHERE  query @@ textsearch
ORDER BY rank DESC LIMIT 10;
                     title                     |        rank
-----------------------------------------------+-------------------
 Neutrinos in the Sun                          | 0.756097569485493
 The Sudbury Neutrino Detector                 | 0.705882361190954
 A MACHO View of Galactic Dark Matter          | 0.668123210574724
 Hot Gas and Dark Matter                       |  0.65655958650282
 The Virgo Cluster: Hot Plasma and Dark Matter | 0.656301290640973
 Rafting for Solar Neutrinos                   | 0.655172410958162
 NGC 4650A: Strange Galaxy and Dark Matter     | 0.650072921219637
 Hot Gas and Dark Matter                       | 0.617195790024749
 Ice Fishing for Cosmic Neutrinos              | 0.615384618911517
 Weak Lensing Distorts the Universe            | 0.450010798361481
</programlisting>
   </para>

   <para>
    Ranking can be expensive since it requires consulting the
    <type>tsvector</type> of each matching document, which can be I/O bound and
    therefore slow. Unfortunately, it is almost impossible to avoid since
    practical queries often result in large numbers of matches.
   </para>

  </sect2>

  <sect2 id="textsearch-headline">
   <title>Highlighting Results</title>

   <para>
    To present search results it is ideal to show a part of each document and
    how it is related to the query. Usually, search engines show fragments of
    the document with marked search terms.  <productname>PostgreSQL</>
    provides a function <function>ts_headline</function> that
    implements this functionality.
   </para>

   <indexterm>
    <primary>ts_headline</primary>
   </indexterm>

   <synopsis>
    ts_headline(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">document</replaceable> <type>text</>, <replaceable class="PARAMETER">query</replaceable> <type>tsquery</> <optional>, <replaceable class="PARAMETER">options</replaceable> <type>text</> </optional>) returns <type>text</>
   </synopsis>

   <para>
    <function>ts_headline</function> accepts a document along
    with a query, and returns an excerpt from
    the document in which terms from the query are highlighted.  The
    configuration to be used to parse the document can be specified by
    <replaceable>config</replaceable>; if <replaceable>config</replaceable>
    is omitted, the
    <varname>default_text_search_config</varname> configuration is used.
   </para>

   <para>
    If an <replaceable>options</replaceable> string is specified it must
    consist of a comma-separated list of one or more
    <replaceable>option</><literal>=</><replaceable>value</> pairs.
    The available options are:

    <itemizedlist  spacing="compact" mark="bullet">
     <listitem>
      <para>
       <literal>StartSel</>, <literal>StopSel</literal>: the strings with which
       query words appearing in the document should be delimited to distinguish
       them from other excerpted words.
      </para>
     </listitem>
     <listitem >
      <para>
       <literal>MaxWords</>, <literal>MinWords</literal>: these numbers
       determine the longest and shortest headlines to output.
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>ShortWord</literal>: words of this length or less will be
       dropped at the start and end of a headline. The default
       value of three eliminates the English articles.
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>HighlightAll</literal>: Boolean flag;  if
       <literal>true</literal> the whole document will be highlighted.
      </para>
     </listitem>
    </itemizedlist>

    Any unspecified options receive these defaults:

<programlisting>
StartSel=&lt;b&gt;, StopSel=&lt;/b&gt;, MaxWords=35, MinWords=15, ShortWord=3, HighlightAll=FALSE
</programlisting>
   </para>

   <para>
    For example:

<programlisting>
SELECT ts_headline('The most common type of search
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                         
------------------------------------------------------------
 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;.

SELECT ts_headline('The most common type of search
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'), 
  'StartSel = &lt;, StopSel = &gt;');
                      ts_headline                      
-------------------------------------------------------
 given &lt;query&gt; terms
 and return them in order of their &lt;similarity&gt; to the
 &lt;query&gt;.
</programlisting>
   </para>

   <para>
    <function>ts_headline</> uses the original document, not a
    <type>tsvector</type> summary, so it can be slow and should be used with
    care.  A typical mistake is to call <function>ts_headline</function> for
    <emphasis>every</emphasis> matching document when only ten documents are
    to be shown. <acronym>SQL</acronym> subqueries can help; here is an
    example:

<programlisting>
SELECT id, ts_headline(body,q), rank
FROM (SELECT id, body, q, ts_rank_cd(ti,q) AS rank
      FROM apod, to_tsquery('stars') q
      WHERE ti @@ q
      ORDER BY rank DESC LIMIT 10) AS foo;
</programlisting>
   </para>

  </sect2>

 </sect1>

 <sect1 id="textsearch-features">
  <title>Additional Features</title>

  <para>
   This section describes additional functions and operators that are
   useful in connection with text search.
  </para>

  <sect2 id="textsearch-manipulate-tsvector">
   <title>Manipulating Documents</title>

   <para>
    <xref linkend="textsearch-parsing-documents"> showed how raw textual
    documents can be converted into <type>tsvector</> values.
    <productname>PostgreSQL</productname> also provides functions and
    operators that can be used to manipulate documents that are already
    in <type>tsvector</> form.
   </para>

   <variablelist>

    <varlistentry>

     <indexterm>
      <primary>tsvector concatenation</primary>
     </indexterm>

     <term>
      <synopsis>
       <type>tsvector</> || <type>tsvector</>
      </synopsis>
     </term>

     <listitem>
      <para>
       The <type>tsvector</> concatenation operator
       returns a vector which combines the lexemes and positional information
       of the two vectors given as arguments.  Positions and weight labels
       are retained during the concatenation.
       Positions appearing in the right-hand vector are offset by the largest
       position mentioned in the left-hand vector, so that the result is
       nearly equivalent to the result of performing <function>to_tsvector</>
       on the concatenation of the two original document strings.  (The
       equivalence is not exact, because any stop-words removed from the
       end of the left-hand argument will not affect the result, whereas
       they would have affected the positions of the lexemes in the
       right-hand argument if textual concatenation were used.)
      </para>

      <para>
       One advantage of using concatenation in the vector form, rather than
       concatenating text before applying <function>to_tsvector</>, is that
       you can use different configurations to parse different sections
       of the document.  Also, because the <function>setweight</> function
       marks all lexemes of the given vector the same way, it is necessary
       to parse the text and do <function>setweight</> before concatenating
       if you want to label different parts of the document with different
       weights.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>

     <indexterm>
      <primary>setweight</primary>
     </indexterm>

     <term>
      <synopsis>
       setweight(<replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>, <replaceable class="PARAMETER">weight</replaceable> <type>"char"</>) returns <type>tsvector</>
      </synopsis>
     </term>

     <listitem>
      <para>
       This function returns a copy of the input vector in which every
       position has been labeled with the given <replaceable>weight</>, either
       <literal>A</literal>, <literal>B</literal>, <literal>C</literal>, or
       <literal>D</literal>.  (<literal>D</literal> is the default for new
       vectors and as such is not displayed on output.)  These labels are
       retained when vectors are concatenated, allowing words from different
       parts of a document to be weighted differently by ranking functions.
      </para>

      <para>
       Note that weight labels apply to <emphasis>positions</>, not
       <emphasis>lexemes</>.  If the input vector has been stripped of
       positions then <function>setweight</> does nothing.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>length(tsvector)</primary>
     </indexterm>

     <term>
      <synopsis>
       length(<replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>) returns <type>integer</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns the number of lexemes stored in the vector.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>

     <indexterm>
      <primary>strip</primary>
     </indexterm>

     <term>
      <synopsis>
       strip(<replaceable class="PARAMETER">vector</replaceable> <type>tsvector</>) returns <type>tsvector</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns a vector which lists the same lexemes as the given vector, but
       which lacks any position or weight information.  While the returned
       vector is much less useful than an unstripped vector for relevance
       ranking, it will usually be much smaller.
      </para>
     </listitem>

    </varlistentry>

   </variablelist>

  </sect2>

  <sect2 id="textsearch-manipulate-tsquery">
   <title>Manipulating Queries</title>

   <para>
    <xref linkend="textsearch-parsing-queries"> showed how raw textual
    queries can be converted into <type>tsquery</> values.
    <productname>PostgreSQL</productname> also provides functions and
    operators that can be used to manipulate queries that are already
    in <type>tsquery</> form.
   </para>

   <variablelist>

    <varlistentry>

     <term>
      <synopsis>
       <type>tsquery</> &amp;&amp; <type>tsquery</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns the AND-combination of the two given queries.
      </para>
     </listitem>

    </varlistentry>

    <varlistentry>

     <term>
      <synopsis>
       <type>tsquery</> || <type>tsquery</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns the OR-combination of the two given queries.
      </para>
     </listitem>

    </varlistentry>

    <varlistentry>

     <term>
      <synopsis>
       !! <type>tsquery</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns the negation (NOT) of the given query.
      </para>
     </listitem>

    </varlistentry>

    <varlistentry>

     <indexterm>
      <primary>numnode</primary>
     </indexterm>

     <term>
      <synopsis>
       numnode(<replaceable class="PARAMETER">query</replaceable> <type>tsquery</>) returns <type>integer</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns the number of nodes (lexemes plus operators) in a
       <type>tsquery</>. This function is useful
       to determine if the <replaceable>query</replaceable> is meaningful
       (returns &gt; 0), or contains only stop words (returns 0).
       Examples:

<programlisting>
SELECT numnode(plainto_tsquery('the any'));
NOTICE:  query contains only stopword(s) or doesn't contain lexeme(s), ignored
 numnode
---------
       0

SELECT numnode('foo &amp; bar'::tsquery);
 numnode
---------
       3
</programlisting>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>

     <indexterm>
      <primary>querytree</primary>
     </indexterm>

     <term>
      <synopsis>
       querytree(<replaceable class="PARAMETER">query</replaceable> <type>tsquery</>) returns <type>text</>
      </synopsis>
     </term>

     <listitem>
      <para>
       Returns the portion of a <type>tsquery</> that can be used for
       searching an index.  This function is useful for detecting
       unindexable queries, for example those containing only stop words
       or only negated terms.  For example:

<programlisting>
SELECT querytree(to_tsquery('!defined'));
 querytree
-----------

</programlisting>
      </para>
     </listitem>
    </varlistentry>

   </variablelist>

   <sect3 id="textsearch-query-rewriting">
    <title>Query Rewriting</title>

    <indexterm zone="textsearch-query-rewriting">
     <primary>ts_rewrite</primary>
    </indexterm>

    <para>
     The <function>ts_rewrite</function> family of functions search a
     given <type>tsquery</> for occurrences of a target
     subquery, and replace each occurrence with another
     substitute subquery.  In essence this operation is a
     <type>tsquery</>-specific version of substring replacement.
     A target and substitute combination can be
     thought of as a <firstterm>query rewrite rule</>.  A collection
     of such rewrite rules can be a powerful search aid.
     For example, you can expand the search using synonyms
     (e.g., <literal>new york</>, <literal>big apple</>, <literal>nyc</>,
     <literal>gotham</>) or narrow the search to direct the user to some hot
     topic.  There is some overlap in functionality between this feature
     and thesaurus dictionaries (<xref linkend="textsearch-thesaurus">).
     However, you can modify a set of rewrite rules on-the-fly without
     reindexing, whereas updating a thesaurus requires reindexing to be
     effective.
    </para>

    <variablelist>

     <varlistentry>

      <term>
       <synopsis>
        ts_rewrite (<replaceable class="PARAMETER">query</replaceable> <type>tsquery</>, <replaceable class="PARAMETER">target</replaceable> <type>tsquery</>, <replaceable class="PARAMETER">substitute</replaceable> <type>tsquery</>) returns <type>tsquery</>
       </synopsis>
      </term>

      <listitem>
       <para>
        This form of <function>ts_rewrite</> simply applies a single
        rewrite rule: <replaceable class="PARAMETER">target</replaceable>
        is replaced by <replaceable class="PARAMETER">substitute</replaceable>
        wherever it appears in <replaceable
        class="PARAMETER">query</replaceable>.  For example:

<programlisting>
SELECT ts_rewrite('a &amp; b'::tsquery, 'a'::tsquery, 'c'::tsquery);
 ts_rewrite
------------
 'b' &amp; 'c'
</programlisting>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>

      <term>
       <synopsis>
        ts_rewrite (<replaceable class="PARAMETER">query</> <type>tsquery</>, <replaceable class="PARAMETER">select</> <type>text</>) returns <type>tsquery</>
       </synopsis>
      </term>

      <listitem>
       <para>
        This form of <function>ts_rewrite</> accepts a starting
        <replaceable>query</> and a SQL <replaceable>select</> command, which
        is given as a text string.  The <replaceable>select</> must yield two
        columns of <type>tsquery</> type.  For each row of the
        <replaceable>select</> result, occurrences of the first column value
        (the target) are replaced by the second column value (the substitute)
        within the current <replaceable>query</> value.  For example:

<programlisting>
CREATE TABLE aliases (t tsquery PRIMARY KEY, s tsquery);
INSERT INTO aliases VALUES('a', 'c');

SELECT ts_rewrite('a &amp; b'::tsquery, 'SELECT t,s FROM aliases');
 ts_rewrite
------------
 'b' &amp; 'c'
</programlisting>
       </para>

       <para>
        Note that when multiple rewrite rules are applied in this way,
        the order of application can be important; so in practice you will
        want the source query to <literal>ORDER BY</> some ordering key.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

    <para>
     Let's consider a real-life astronomical example. We'll expand query
     <literal>supernovae</literal> using table-driven rewriting rules:

<programlisting>
CREATE TABLE aliases (t tsquery primary key, s tsquery);
INSERT INTO aliases VALUES(to_tsquery('supernovae'), to_tsquery('supernovae|sn'));

SELECT ts_rewrite(to_tsquery('supernovae &amp; crab'), 'SELECT * FROM aliases');
           ts_rewrite            
---------------------------------
 'crab' &amp; ( 'supernova' | 'sn' )
</programlisting>

     We can change the rewriting rules just by updating the table:

<programlisting>
UPDATE aliases SET s = to_tsquery('supernovae|sn &amp; !nebulae') WHERE t = to_tsquery('supernovae');

SELECT ts_rewrite(to_tsquery('supernovae &amp; crab'), 'SELECT * FROM aliases');
                 ts_rewrite                  
---------------------------------------------
 'crab' &amp; ( 'supernova' | 'sn' &amp; !'nebula' )
</programlisting>
    </para>

    <para>
     Rewriting can be slow when there are many rewriting rules, since it
     checks every rule for a possible hit. To filter out obvious non-candidate
     rules we can use the containment operators for the <type>tsquery</type>
     type. In the example below, we select only those rules which might match
     the original query:

<programlisting>
SELECT ts_rewrite('a &amp; b'::tsquery,
                  'SELECT t,s FROM aliases WHERE ''a &amp; b''::tsquery @&gt; t');
 ts_rewrite
------------
 'b' &amp; 'c'
</programlisting>
    </para>

   </sect3>

  </sect2>

  <sect2 id="textsearch-update-triggers">
   <title>Triggers for Automatic Updates</title>

   <indexterm>
    <primary>trigger</primary>
    <secondary>for updating a derived tsvector column</secondary>
   </indexterm>

   <para>
    When using a separate column to store the <type>tsvector</> representation
    of your documents, it is necessary to create a trigger to update the
    <type>tsvector</> column when the document content columns change.
    Two built-in trigger functions are available for this, or you can write
    your own.
   </para>

   <synopsis>
    tsvector_update_trigger(<replaceable class="PARAMETER">tsvector_column_name</replaceable>, <replaceable class="PARAMETER">config_name</replaceable>, <replaceable class="PARAMETER">text_column_name</replaceable> <optional>, ... </optional>)
    tsvector_update_trigger_column(<replaceable class="PARAMETER">tsvector_column_name</replaceable>, <replaceable class="PARAMETER">config_column_name</replaceable>, <replaceable class="PARAMETER">text_column_name</replaceable> <optional>, ... </optional>)
   </synopsis>

   <para>
    These trigger functions automatically compute a <type>tsvector</>
    column from one or more textual columns, under the control of
    parameters specified in the <command>CREATE TRIGGER</> command.
    An example of their use is:

<programlisting>
CREATE TABLE messages (
    title       text,
    body        text,
    tsv         tsvector
);

CREATE TRIGGER tsvectorupdate BEFORE INSERT OR UPDATE
ON messages FOR EACH ROW EXECUTE PROCEDURE
tsvector_update_trigger(tsv, 'pg_catalog.english', title, body);

INSERT INTO messages VALUES('title here', 'the body text is here');

SELECT * FROM messages;
   title    |         body          |            tsv             
------------+-----------------------+----------------------------
 title here | the body text is here | 'bodi':4 'text':5 'titl':1

SELECT title, body FROM messages WHERE tsv @@ to_tsquery('title &amp; body');
   title    |         body          
------------+-----------------------
 title here | the body text is here
</programlisting>

    Having created this trigger, any change in <structfield>title</> or
    <structfield>body</> will automatically be reflected into
    <structfield>tsv</>, without the application having to worry about it.
   </para>

   <para>
    The first trigger argument must be the name of the <type>tsvector</>
    column to be updated.  The second argument specifies the text search
    configuration to be used to perform the conversion.  For
    <function>tsvector_update_trigger</>, the configuration name is simply
    given as the second trigger argument.  It must be schema-qualified as
    shown above, so that the trigger behavior will not change with changes
    in <varname>search_path</>.  For
    <function>tsvector_update_trigger_column</>, the second trigger argument
    is the name of another table column, which must be of type
    <type>regconfig</>.  This allows a per-row selection of configuration
    to be made.  The remaining argument(s) are the names of textual columns
    (of type <type>text</>, <type>varchar</>, or <type>char</>).  These
    will be included in the document in the order given.  NULL values will
    be skipped (but the other columns will still be indexed).
   </para>

   <para>
    A limitation of the built-in triggers is that they treat all the
    input columns alike.  To process columns differently &mdash; for
    example, to weight title differently from body &mdash; it is necessary
    to write a custom trigger.  Here is an example using
    <application>PL/pgSQL</application> as the trigger language:

<programlisting>
CREATE FUNCTION messages_trigger() RETURNS trigger AS $$
begin
  new.tsv :=
     setweight(to_tsvector('pg_catalog.english', coalesce(new.title,'')), 'A') ||
     setweight(to_tsvector('pg_catalog.english', coalesce(new.body,'')), 'D');
  return new;
end
$$ LANGUAGE plpgsql;

CREATE TRIGGER tsvectorupdate BEFORE INSERT OR UPDATE
ON messages FOR EACH ROW EXECUTE PROCEDURE messages_trigger();
</programlisting>
   </para>

   <para>
    Keep in mind that it is important to specify the configuration name
    explicitly when creating <type>tsvector</> values inside triggers,
    so that the column's contents will not be affected by changes to
    <varname>default_text_search_config</>.  Failure to do this is likely to
    lead to problems such as search results changing after a dump and reload.
   </para>

  </sect2>

  <sect2 id="textsearch-statistics">
   <title>Gathering Document Statistics</title>

   <indexterm>
    <primary>ts_stat</primary>
   </indexterm>

   <para>
    The function <function>ts_stat</> is useful for checking your
    configuration and for finding stop-word candidates.
   </para>

   <synopsis>
    ts_stat(<replaceable class="PARAMETER">sqlquery</replaceable> <type>text</>, <optional> <replaceable class="PARAMETER">weights</replaceable> <type>text</>, </optional> OUT <replaceable class="PARAMETER">word</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">ndoc</replaceable> <type>integer</>, OUT <replaceable class="PARAMETER">nentry</replaceable> <type>integer</>) returns <type>setof record</>
   </synopsis>

   <para>
    <replaceable>sqlquery</replaceable> is a text value containing a SQL
    query which must return a single <type>tsvector</type> column.
    <function>ts_stat</> executes the query and returns statistics about
    each distinct lexeme (word) contained in the <type>tsvector</type>
    data.  The columns returned are

    <itemizedlist  spacing="compact" mark="bullet">
     <listitem>
      <para>
       <replaceable>word</> <type>text</> &mdash; the value of a lexeme
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>ndoc</> <type>integer</> &mdash; number of documents
       (<type>tsvector</>s) the word occurred in
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>nentry</> <type>integer</> &mdash; total number of
       occurrences of the word
      </para>
     </listitem>
    </itemizedlist>

    If <replaceable>weights</replaceable> is supplied, only occurrences
    having one of those weights are counted.
   </para>

   <para>
    For example, to find the ten most frequent words in a document collection:

<programlisting>
SELECT * FROM ts_stat('SELECT vector FROM apod')
ORDER BY nentry DESC, ndoc DESC, word
LIMIT 10;
</programlisting>

    The same, but counting only word occurrences with weight <literal>A</>
    or <literal>B</>:

<programlisting>
SELECT * FROM ts_stat('SELECT vector FROM apod', 'ab')
ORDER BY nentry DESC, ndoc DESC, word
LIMIT 10;
</programlisting>
   </para>

  </sect2>

 </sect1>

 <sect1 id="textsearch-parsers">
  <title>Parsers</title>

  <para>
   Text search parsers are responsible for splitting raw document text
   into <firstterm>tokens</> and identifying each token's type, where
   the set of possible types is defined by the parser itself.
   Note that a parser does not modify the text at all &mdash; it simply
   identifies plausible word boundaries.  Because of this limited scope,
   there is less need for application-specific custom parsers than there is
   for custom dictionaries.  At present <productname>PostgreSQL</productname>
   provides just one built-in parser, which has been found to be useful for a
   wide range of applications.
  </para>

  <para>
   The built-in parser is named <literal>pg_catalog.default</>.
   It recognizes 23 token types:
  </para>

  <table id="textsearch-default-parser">
   <title>Default Parser's Token Types</title>
   <tgroup cols="3">
    <thead>
     <row>
      <entry>Alias</entry>
      <entry>Description</entry>
      <entry>Example</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry><literal>asciiword</></entry>
      <entry>Word, all ASCII letters</entry>
      <entry><literal>elephant</literal></entry>
     </row>
     <row>
      <entry><literal>word</></entry>
      <entry>Word, all letters</entry>
      <entry><literal>ma&ntilde;ana</literal></entry>
     </row>
     <row>
      <entry><literal>numword</></entry>
      <entry>Word, letters and digits</entry>
      <entry><literal>beta1</literal></entry>
     </row>
     <row>
      <entry><literal>asciihword</></entry>
      <entry>Hyphenated word, all ASCII</entry>
      <entry><literal>up-to-date</literal></entry>
     </row>
     <row>
      <entry><literal>hword</></entry>
      <entry>Hyphenated word, all letters</entry>
      <entry><literal>l&oacute;gico-matem&aacute;tica</literal></entry>
     </row>
     <row>
      <entry><literal>numhword</></entry>
      <entry>Hyphenated word, letters and digits</entry>
      <entry><literal>postgresql-beta1</literal></entry>
     </row>
     <row>
      <entry><literal>hword_asciipart</></entry>
      <entry>Hyphenated word part, all ASCII</entry>
      <entry><literal>postgresql</literal> in the context <literal>postgresql-beta1</literal></entry>
     </row>
     <row>
      <entry><literal>hword_part</></entry>
      <entry>Hyphenated word part, all letters</entry>
      <entry><literal>l&oacute;gico</literal> or <literal>matem&aacute;tica</literal>
       in the context <literal>l&oacute;gico-matem&aacute;tica</literal></entry>
     </row>
     <row>
      <entry><literal>hword_numpart</></entry>
      <entry>Hyphenated word part, letters and digits</entry>
      <entry><literal>beta1</literal> in the context
       <literal>postgresql-beta1</literal></entry>
     </row>
     <row>
      <entry><literal>email</></entry>
      <entry>Email address</entry>
      <entry><literal>foo@example.com</literal></entry>
     </row>
     <row>
      <entry><literal>protocol</></entry>
      <entry>Protocol head</entry>
      <entry><literal>http://</literal></entry>
     </row>
     <row>
      <entry><literal>url</></entry>
      <entry>URL</entry>
      <entry><literal>example.com/stuff/index.html</literal></entry>
     </row>
     <row>
      <entry><literal>host</></entry>
      <entry>Host</entry>
      <entry><literal>example.com</literal></entry>
     </row>
     <row>
      <entry><literal>url_path</></entry>
      <entry>URL path</entry>
      <entry><literal>/stuff/index.html</literal>, in the context of a URL</entry>
     </row>
     <row>
      <entry><literal>file</></entry>
      <entry>File or path name</entry>
      <entry><literal>/usr/local/foo.txt</literal>, if not within a URL</entry>
     </row>
     <row>
      <entry><literal>sfloat</></entry>
      <entry>Scientific notation</entry>
      <entry><literal>-1.234e56</literal></entry>
     </row>
     <row>
      <entry><literal>float</></entry>
      <entry>Decimal notation</entry>
      <entry><literal>-1.234</literal></entry>
     </row>
     <row>
      <entry><literal>int</></entry>
      <entry>Signed integer</entry>
      <entry><literal>-1234</literal></entry>
     </row>
     <row>
      <entry><literal>uint</></entry>
      <entry>Unsigned integer</entry>
      <entry><literal>1234</literal></entry>
     </row>
     <row>
      <entry><literal>version</></entry>
      <entry>Version number</entry>
      <entry><literal>8.3.0</literal></entry>
     </row>
     <row>
      <entry><literal>tag</></entry>
      <entry>XML tag</entry>
      <entry><literal>&lt;a href="dictionaries.html"&gt;</literal></entry>
     </row>
     <row>
      <entry><literal>entity</></entry>
      <entry>XML entity</entry>
      <entry><literal>&amp;amp;</literal></entry>
     </row>
     <row>
      <entry><literal>blank</></entry>
      <entry>Space symbols</entry>
      <entry>(any whitespace or punctuation not otherwise recognized)</entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <note>
   <para>
    The parser's notion of a <quote>letter</> is determined by the server's
    locale setting, specifically <varname>lc_ctype</>.  Words containing
    only the basic ASCII letters are reported as a separate token type,
    since it is sometimes useful to distinguish them.  In most European
    languages, token types <literal>word</> and <literal>asciiword</>
    should always be treated alike.
   </para>
  </note>

  <para>
   It is possible for the parser to produce overlapping tokens from the same
   piece of text.  As an example, a hyphenated word will be reported both
   as the entire word and as each component:

<programlisting>
SELECT alias, description, token FROM ts_debug('foo-bar-beta1');
      alias      |               description                |     token     
-----------------+------------------------------------------+---------------
 numhword        | Hyphenated word, letters and digits      | foo-bar-beta1
 hword_asciipart | Hyphenated word part, all ASCII          | foo
 blank           | Space symbols                            | -
 hword_asciipart | Hyphenated word part, all ASCII          | bar
 blank           | Space symbols                            | -
 hword_numpart   | Hyphenated word part, letters and digits | beta1
</programlisting>

   This behavior is desirable since it allows searches to work for both
   the whole compound word and for components.  Here is another
   instructive example:

<programlisting>
SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.html');
  alias   |  description  |            token             
----------+---------------+------------------------------
 protocol | Protocol head | http://
 url      | URL           | example.com/stuff/index.html
 host     | Host          | example.com
 url_path | URL path      | /stuff/index.html
</programlisting>
  </para>

 </sect1>

 <sect1 id="textsearch-dictionaries">
  <title>Dictionaries</title>

  <para>
   Dictionaries are used to eliminate words that should not be considered in a
   search (<firstterm>stop words</>), and to <firstterm>normalize</> words so
   that different derived forms of the same word will match.  A successfully
   normalized word is called a <firstterm>lexeme</>.  Aside from
   improving search quality, normalization and removal of stop words reduce the
   size of the <type>tsvector</type> representation of a document, thereby
   improving performance.  Normalization does not always have linguistic meaning
   and usually depends on application semantics.
  </para>

  <para>
   Some examples of normalization:

   <itemizedlist  spacing="compact" mark="bullet">

    <listitem>
     <para>
      Linguistic - Ispell dictionaries try to reduce input words to a
      normalized form; stemmer dictionaries remove word endings
     </para>
    </listitem>
    <listitem>
     <para>
      <acronym>URL</acronym> locations can be canonicalized to make
      equivalent URLs match:

      <itemizedlist  spacing="compact" mark="bullet">
       <listitem>
        <para>
         http://www.pgsql.ru/db/mw/index.html
        </para>
       </listitem>
       <listitem>
        <para>
         http://www.pgsql.ru/db/mw/
        </para>
       </listitem>
       <listitem>
        <para>
         http://www.pgsql.ru/db/../db/mw/index.html
        </para>
       </listitem>
      </itemizedlist>
     </para>
    </listitem>
    <listitem>
     <para>
      Color names can be replaced by their hexadecimal values, e.g.,
      <literal>red, green, blue, magenta -> FF0000, 00FF00, 0000FF, FF00FF</literal>
     </para>
    </listitem>
    <listitem>
     <para>
      If indexing numbers, we can
      remove some fractional digits to reduce the range of possible
      numbers, so for example <emphasis>3.14</emphasis>159265359,
      <emphasis>3.14</emphasis>15926, <emphasis>3.14</emphasis> will be the same
      after normalization if only two digits are kept after the decimal point.
     </para>
    </listitem>
   </itemizedlist>

  </para>

  <para>
   A dictionary is a program that accepts a token as
   input and returns:
   <itemizedlist  spacing="compact" mark="bullet">
    <listitem>
     <para>
      an array of lexemes if the input token is known to the dictionary
      (notice that one token can produce more than one lexeme)
     </para>
    </listitem>
    <listitem>
     <para>
      an empty array if the dictionary knows the token, but it is a stop word
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>NULL</literal> if the dictionary does not recognize the input token
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   <productname>PostgreSQL</productname> provides predefined dictionaries for
   many languages.  There are also several predefined templates that can be
   used to create new dictionaries with custom parameters.  Each predefined
   dictionary template is described below.  If no existing
   template is suitable, it is possible to create new ones; see the
   <filename>contrib/</> area of the <productname>PostgreSQL</> distribution
   for examples.
  </para>

  <para>
   A text search configuration binds a parser together with a set of
   dictionaries to process the parser's output tokens.  For each token
   type that the parser can return, a separate list of dictionaries is
   specified by the configuration.  When a token of that type is found
   by the parser, each dictionary in the list is consulted in turn,
   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.
   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
   a <application>Snowball</> stemmer or <literal>simple</>, which
   recognizes everything.  For example, for an astronomy-specific search
   (<literal>astro_en</literal> configuration) one could bind token type
   <type>asciiword</type> (ASCII word) to a synonym dictionary of astronomical
   terms, a general English dictionary and a <application>Snowball</> English
   stemmer:

<programlisting>
ALTER TEXT SEARCH CONFIGURATION astro_en
    ADD MAPPING FOR asciiword WITH astrosyn, english_ispell, english_stem;
</programlisting>
  </para>

  <sect2 id="textsearch-stopwords">
   <title>Stop Words</title>

   <para>
    Stop words are words that are very common, appear in almost every
    document, and have no discrimination value. Therefore, they can be ignored
    in the context of full text searching. For example, every English text
    contains words like <literal>a</literal> and <literal>the</>, so it is
    useless to store them in an index.  However, stop words do affect the
    positions in <type>tsvector</type>, which in turn affect ranking:

<programlisting>
SELECT to_tsvector('english','in the list of stop words');
        to_tsvector
----------------------------
 'list':3 'stop':5 'word':6
</programlisting>

    The missing positions 1,2,4 are because of stop words.  Ranks
    calculated for documents with and without stop words are quite different:

<programlisting>
SELECT ts_rank_cd (to_tsvector('english','in the list of stop words'), to_tsquery('list &amp; stop'));
 ts_rank_cd
------------
       0.05

SELECT ts_rank_cd (to_tsvector('english','list stop words'), to_tsquery('list &amp; stop'));
 ts_rank_cd
------------
        0.1
</programlisting>

   </para>

   <para>
    It is up to the specific dictionary how it treats stop words. For example,
    <literal>ispell</literal> dictionaries first normalize words and then
    look at the list of stop words, while <literal>Snowball</literal> stemmers
    first check the list of stop words. The reason for the different
    behavior is an attempt to decrease noise.
   </para>

  </sect2>

  <sect2 id="textsearch-simple-dictionary">
   <title>Simple Dictionary</title>

   <para>
    The <literal>simple</> dictionary template operates by converting the
    input token to lower case and checking it against a file of stop words.
    If it is found in the file then an empty array is returned, causing
    the token to be discarded.  If not, the lower-cased form of the word
    is returned as the normalized lexeme.  Alternatively, the dictionary
    can be configured to report non-stop-words as unrecognized, allowing
    them to be passed on to the next dictionary in the list.
   </para>

   <para>
    Here is an example of a dictionary definition using the <literal>simple</>
    template:

<programlisting>
CREATE TEXT SEARCH DICTIONARY public.simple_dict (
    TEMPLATE = pg_catalog.simple,
    STOPWORDS = english
);
</programlisting>

    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 
    <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).
    The file format is simply a list
    of words, one per line.  Blank lines and trailing spaces are ignored,
    and upper case is folded to lower case, but no other processing is done
    on the file contents.
   </para>

   <para>
    Now we can test our dictionary:

<programlisting>
SELECT ts_lexize('public.simple_dict','YeS');
 ts_lexize
-----------
 {yes}

SELECT ts_lexize('public.simple_dict','The');
 ts_lexize
-----------
 {}
</programlisting>
   </para>

   <para>
    We can also choose to return <literal>NULL</>, instead of the lower-cased
    word, if it is not found in the stop words file.  This behavior is
    selected by setting the dictionary's <literal>Accept</> parameter to
    <literal>false</>.  Continuing the example:

<programlisting>
ALTER TEXT SEARCH DICTIONARY public.simple_dict ( Accept = false );

SELECT ts_lexize('public.simple_dict','YeS');
 ts_lexize
-----------


SELECT ts_lexize('public.simple_dict','The');
 ts_lexize
-----------
 {}
</programlisting>
   </para>

   <para>
    With the default setting of <literal>Accept</> = <literal>true</>,
    it is only useful to place a <literal>simple</> dictionary at the end
    of a list of dictionaries, since it will never pass on any token to
    a following dictionary.  Conversely, <literal>Accept</> = <literal>false</>
    is only useful when there is at least one following dictionary.
   </para>

   <caution>
    <para>
     Most types of dictionaries rely on configuration files, such as files of
     stop words.  These files <emphasis>must</> be stored in UTF-8 encoding.
     They will be translated to the actual database encoding, if that is
     different, when they are read into the server.
    </para>
   </caution>

   <caution>
    <para>
     Normally, a database session will read a dictionary configuration file
     only once, when it is first used within the session.  If you modify a
     configuration file and want to force existing sessions to pick up the
     new contents, issue an <command>ALTER TEXT SEARCH DICTIONARY</> command
     on the dictionary.  This can be a <quote>dummy</> update that doesn't
     actually change any parameter values.
    </para>
   </caution>

  </sect2>

  <sect2 id="textsearch-synonym-dictionary">
   <title>Synonym Dictionary</title>

   <para>
    This dictionary template is used to create dictionaries that replace a
    word with a synonym. Phrases are not supported (use the thesaurus
    template (<xref linkend="textsearch-thesaurus">) for that).  A synonym
    dictionary can be used to overcome linguistic problems, for example, to
    prevent an English stemmer dictionary from reducing the word 'Paris' to
    'pari'.  It is enough to have a <literal>Paris paris</literal> line in the
    synonym dictionary and put it before the <literal>english_stem</> dictionary:

<programlisting>
SELECT * FROM ts_debug('english', 'Paris');
   alias   |   description   | token |  dictionaries  |  dictionary  | lexemes 
-----------+-----------------+-------+----------------+--------------+---------
 asciiword | Word, all ASCII | Paris | {english_stem} | english_stem | {pari}

CREATE TEXT SEARCH DICTIONARY my_synonym (
    TEMPLATE = synonym,
    SYNONYMS = my_synonyms
);

ALTER TEXT SEARCH CONFIGURATION english
    ALTER MAPPING FOR asciiword WITH my_synonym, english_stem;

SELECT * FROM ts_debug('english', 'Paris');
   alias   |   description   | token |       dictionaries        | dictionary | lexemes 
-----------+-----------------+-------+---------------------------+------------+---------
 asciiword | Word, all ASCII | Paris | {my_synonym,english_stem} | my_synonym | {paris}
</programlisting>
   </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,
    and upper case is folded to lower case.
   </para>

  </sect2>

  <sect2 id="textsearch-thesaurus">
   <title>Thesaurus Dictionary</title>

   <para>
    A thesaurus dictionary (sometimes abbreviated as <acronym>TZ</acronym>) is
    a collection of words that includes information about the relationships
    of words and phrases, i.e., broader terms (<acronym>BT</acronym>), narrower
    terms (<acronym>NT</acronym>), preferred terms, non-preferred terms, related
    terms, etc.
   </para>

   <para>
    Basically a thesaurus dictionary replaces all non-preferred terms by one
    preferred term and, optionally, preserves the original terms for indexing
    as well.  <productname>PostgreSQL</>'s current implementation of the
    thesaurus dictionary is an extension of the synonym dictionary with added
    <firstterm>phrase</firstterm> support.  A thesaurus dictionary requires
    a configuration file of the following format:

<programlisting>
# this is a comment
sample word(s) : indexed word(s)
more sample word(s) : more indexed word(s)
...
</programlisting>

    where  the colon (<symbol>:</symbol>) symbol acts as a delimiter between a
    a phrase and its replacement.
   </para>

   <para>
    A thesaurus dictionary uses a <firstterm>subdictionary</firstterm> (which
    is specified in the dictionary's configuration) to normalize the input
    text before checking for phrase matches. It is only possible to select one
    subdictionary.  An error is reported if the subdictionary fails to
    recognize a word. In that case, you should remove the use of the word or
    teach the subdictionary about it.  You can place an asterisk
    (<symbol>*</symbol>) at the beginning of an indexed word to skip applying
    the subdictionary to it, but all sample words <emphasis>must</> be known
    to the subdictionary.
   </para>

   <para>
    The thesaurus dictionary chooses the longest match if there are multiple
    phrases matching the input, and ties are broken by using the last
    definition.
   </para>

   <para>
    Specific stop words recognized by the subdictionary cannot be
    specified;  instead use <literal>?</> to mark the location where any
    stop word can appear.  For example, assuming that <literal>a</> and
    <literal>the</> are stop words according to the subdictionary:

<programlisting>
? one ? two : swsw
</programlisting>

    matches <literal>a one the two</> and <literal>the one a two</>;
    both would be replaced by <literal>swsw</>.
   </para>

   <para>
    Since a thesaurus dictionary has the capability to recognize phrases it
    must remember its state and interact with the parser. A thesaurus dictionary
    uses these assignments to check if it should handle the next word or stop
    accumulation.  The thesaurus dictionary must be configured
    carefully. For example, if the thesaurus dictionary is assigned to handle
    only the <literal>asciiword</literal> token, then a thesaurus dictionary
    definition like <literal>one 7</> will not work since token type
    <literal>uint</literal> is not assigned to the thesaurus dictionary.
   </para>

   <caution>
    <para>
     Thesauruses are used during indexing so any change in the thesaurus
     dictionary's parameters <emphasis>requires</emphasis> reindexing.
     For most other dictionary types, small changes such as adding or
     removing stopwords does not force reindexing.
    </para>
   </caution>

  <sect3 id="textsearch-thesaurus-config">
   <title>Thesaurus Configuration</title>

   <para>
    To define a new thesaurus dictionary, use the <literal>thesaurus</>
    template.  For example:

<programlisting>
CREATE TEXT SEARCH DICTIONARY thesaurus_simple (
    TEMPLATE = thesaurus,
    DictFile = mythesaurus,
    Dictionary = pg_catalog.english_stem
);
</programlisting>

    Here:
    <itemizedlist  spacing="compact" mark="bullet">
     <listitem>
      <para>
       <literal>thesaurus_simple</literal> is the new dictionary's name
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>mythesaurus</literal> is the base name of the thesaurus
       configuration file.
       (Its full name will be <filename>$SHAREDIR/tsearch_data/mythesaurus.ths</>,
       where <literal>$SHAREDIR</> means the installation shared-data
       directory.)
      </para>
     </listitem>
     <listitem>
      <para>
       <literal>pg_catalog.english_stem</literal> is the subdictionary (here,
       a Snowball English stemmer) to use for thesaurus normalization.
       Notice that the subdictionary will have its own
       configuration (for example, stop words), which is not shown here.
      </para>
     </listitem>
    </itemizedlist>

    Now it is possible to bind the thesaurus dictionary <literal>thesaurus_simple</literal>
    to the desired token types in a configuration, for example:

<programlisting>
ALTER TEXT SEARCH CONFIGURATION russian
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart WITH thesaurus_simple;
</programlisting>
   </para>

  </sect3>

  <sect3 id="textsearch-thesaurus-examples">
   <title>Thesaurus Example</title>

   <para>
    Consider a simple astronomical thesaurus <literal>thesaurus_astro</literal>,
    which contains some astronomical word combinations:

<programlisting>
supernovae stars : sn
crab nebulae : crab
</programlisting>

    Below we create a dictionary and bind some token types to
    an astronomical thesaurus and English stemmer:

<programlisting>
CREATE TEXT SEARCH DICTIONARY thesaurus_astro (
    TEMPLATE = thesaurus,
    DictFile = thesaurus_astro,
    Dictionary = english_stem
);

ALTER TEXT SEARCH CONFIGURATION russian
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart WITH thesaurus_astro, english_stem;
</programlisting>

    Now we can see how it works.
    <function>ts_lexize</function> is not very useful for testing a thesaurus,
    because it treats its input as a single token.  Instead we can use
    <function>plainto_tsquery</function> and <function>to_tsvector</function>
    which will break their input strings into multiple tokens:

<programlisting>
SELECT plainto_tsquery('supernova star');
 plainto_tsquery
-----------------
 'sn'

SELECT to_tsvector('supernova star');
 to_tsvector
-------------
 'sn':1
</programlisting>

    In principle, one can use <function>to_tsquery</function> if you quote
    the argument:

<programlisting>
SELECT to_tsquery('''supernova star''');
 to_tsquery
------------
 'sn'
</programlisting>

    Notice that <literal>supernova star</literal> matches <literal>supernovae
    stars</literal> in <literal>thesaurus_astro</literal> because we specified
    the <literal>english_stem</literal> stemmer in the thesaurus definition.
    The stemmer removed the <literal>e</> and <literal>s</>.
   </para>

   <para>
    To index the original phrase as well as the substitute, just include it
    in the right-hand part of the definition:

<programlisting>
supernovae stars : sn supernovae stars

SELECT plainto_tsquery('supernova star');
       plainto_tsquery
-----------------------------
 'sn' &amp; 'supernova' &amp; 'star'
</programlisting>
   </para>

  </sect3>

  </sect2>

  <sect2 id="textsearch-ispell-dictionary">
   <title><application>Ispell</> Dictionary</title>

   <para>
    The <application>Ispell</> dictionary template supports
    <firstterm>morphological dictionaries</>, which can normalize many
    different linguistic forms of a word into the same lexeme.  For example,
    an English <application>Ispell</> dictionary can match all declensions and
    conjugations of the search term <literal>bank</literal>, e.g.
    <literal>banking</>, <literal>banked</>, <literal>banks</>,
    <literal>banks'</>, and <literal>bank's</>.
   </para>

   <para>
    The standard <productname>PostgreSQL</productname> distribution does
    not include any <application>Ispell</> configuration files.
    Dictionaries for a large number of languages are available from <ulink
    url="http://ficus-www.cs.ucla.edu/geoff/ispell.html">Ispell</ulink>.
    Also, some more modern dictionary file formats are supported &mdash; <ulink
    url="http://en.wikipedia.org/wiki/MySpell">MySpell</ulink> (OO &lt; 2.0.1)
    and <ulink url="http://sourceforge.net/projects/hunspell">Hunspell</ulink>
    (OO &gt;= 2.0.2).  A large list of dictionaries is available on the <ulink
    url="http://wiki.services.openoffice.org/wiki/Dictionaries">OpenOffice
    Wiki</ulink>.
   </para>

   <para>
    To create an <application>Ispell</> dictionary, use the built-in
    <literal>ispell</literal> template and specify several parameters:
   </para>

<programlisting>
CREATE TEXT SEARCH DICTIONARY english_ispell (
    TEMPLATE = ispell,
    DictFile = english,
    AffFile = english,
    StopWords = english
);
</programlisting>

   <para>
    Here, <literal>DictFile</>, <literal>AffFile</>, and <literal>StopWords</>
    specify the base names of the dictionary, affixes, and stop-words files.
    The stop-words file has the same format explained above for the
    <literal>simple</> dictionary type.  The format of the other files is
    not specified here but is available from the above-mentioned web sites.
   </para>

   <para>
    Ispell dictionaries usually recognize a limited set of words, so they
    should be followed by another broader dictionary; for
    example, a Snowball dictionary, which recognizes everything.
   </para>

   <para>
    Ispell dictionaries support splitting compound words.
    This is a nice feature and
    <productname>PostgreSQL</productname> supports it.
    Notice that the affix file should specify a special flag using the
    <literal>compoundwords controlled</literal> statement that marks dictionary
    words that can participate in compound formation:

<programlisting>
compoundwords  controlled z
</programlisting>

    Here are some examples for the Norwegian language:

<programlisting>
SELECT ts_lexize('norwegian_ispell', 'overbuljongterningpakkmesterassistent');
   {over,buljong,terning,pakk,mester,assistent}
SELECT ts_lexize('norwegian_ispell', 'sjokoladefabrikk');
   {sjokoladefabrikk,sjokolade,fabrikk}
</programlisting>
   </para>

   <note>
    <para>
     <application>MySpell</> does not support compound words.
     <application>Hunspell</> has sophisticated support for compound words. At
     present, <productname>PostgreSQL</productname> implements only the basic
     compound word operations of Hunspell.
    </para>
   </note>

  </sect2>

  <sect2 id="textsearch-snowball-dictionary">
   <title><application>Snowball</> Dictionary</title>

   <para>
    The <application>Snowball</> dictionary template is based on the project
    of Martin Porter, inventor of the popular Porter's stemming algorithm
    for the English language.  Snowball now provides stemming algorithms for
    many languages (see the <ulink url="http://snowball.tartarus.org">Snowball
    site</ulink> for more information).  Each algorithm understands how to
    reduce common variant forms of words to a base, or stem, spelling within
    its language.  A Snowball dictionary requires a <literal>language</>
    parameter to identify which stemmer to use, and optionally can specify a
    <literal>stopword</> file name that gives a list of words to eliminate.
    (<productname>PostgreSQL</productname>'s standard stopword lists are also
    provided by the Snowball project.)
    For example, there is a built-in definition equivalent to

<programlisting>
CREATE TEXT SEARCH DICTIONARY english_stem (
    TEMPLATE = snowball,
    Language = english,
    StopWords = english
);
</programlisting>

    The stopword file format is the same as already explained.
   </para>

   <para>
    A <application>Snowball</> dictionary recognizes everything, whether
    or not it is able to simplify the word, so it should be placed
    at the end of the dictionary list. It is useless to have it
    before any other dictionary because a token will never pass through it to
    the next dictionary.
   </para>

  </sect2>

 </sect1>

 <sect1 id="textsearch-configuration">
  <title>Configuration Example</title>

   <para>
    A text search configuration specifies all options necessary to transform a
    document into a <type>tsvector</type>: the parser to use to break text
    into tokens, and the dictionaries to use to transform each token into a
    lexeme.  Every call of
    <function>to_tsvector</function> or <function>to_tsquery</function>
    needs a text search configuration to perform its processing.
    The configuration parameter
    <xref linkend="guc-default-text-search-config">
    specifies the name of the default configuration, which is the
    one used by text search functions if an explicit configuration
    parameter is omitted.
    It can be set in <filename>postgresql.conf</filename>, or set for an
    individual session using the <command>SET</> command.
   </para>

   <para>
    Several predefined text search configurations are available, and
    you can create custom configurations easily.  To facilitate management
    of text search objects, a set of <acronym>SQL</acronym> commands
    is available, and there are several <application>psql</application> commands that display information
    about text search objects (<xref linkend="textsearch-psql">).
   </para>

   <para>
    As an example, we will create a configuration
    <literal>pg</literal>, starting from a duplicate of the built-in
    <literal>english</> configuration.

<programlisting>
CREATE TEXT SEARCH CONFIGURATION public.pg ( COPY = pg_catalog.english );
</programlisting>
   </para>

   <para>
    We will use a PostgreSQL-specific synonym list
    and store it in <filename>$SHAREDIR/tsearch_data/pg_dict.syn</filename>.
    The file contents look like:

<programlisting>
postgres    pg
pgsql       pg
postgresql  pg
</programlisting>

    We define the synonym dictionary like this:

<programlisting>
CREATE TEXT SEARCH DICTIONARY pg_dict (
    TEMPLATE = synonym,
    SYNONYMS = pg_dict
);
</programlisting>

    Next we register the <productname>Ispell</> dictionary
    <literal>english_ispell</literal>, which has its own configuration files:

<programlisting>
CREATE TEXT SEARCH DICTIONARY english_ispell (
    TEMPLATE = ispell,
    DictFile = english,
    AffFile = english,
    StopWords = english
);
</programlisting>

    Now we can set up the mappings for words in configuration
    <literal>pg</>:

<programlisting>
ALTER TEXT SEARCH CONFIGURATION pg
    ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
                      word, hword, hword_part
    WITH pg_dict, english_ispell, english_stem;
</programlisting>

    We choose not to index or search some token types that the built-in
    configuration does handle:

<programlisting>
ALTER TEXT SEARCH CONFIGURATION pg
    DROP MAPPING FOR email, url, url_path, sfloat, float;
</programlisting>
   </para>

   <para>
    Now we can test our configuration:

<programlisting>
SELECT * FROM ts_debug('public.pg', '
PostgreSQL, the highly scalable, SQL compliant, open source object-relational
database management system, is now undergoing beta testing of the next
version of our software.
');
</programlisting>
   </para>

   <para>
    The next step is to set the session to use the new configuration, which was
    created in the <literal>public</> schema:

<programlisting>
=&gt; \dF
   List of text search configurations
 Schema  | Name | Description
---------+------+-------------
 public  | pg   |

SET default_text_search_config = 'public.pg';
SET

SHOW default_text_search_config;
 default_text_search_config
----------------------------
 public.pg
</programlisting>
  </para>

 </sect1>

 <sect1 id="textsearch-debugging">
  <title>Testing and Debugging Text Search</title>

  <para>
   The behavior of a custom text search configuration can easily become
   complicated enough to be confusing or undesirable.  The functions described
   in this section are useful for testing text search objects.  You can
   test a complete configuration, or test parsers and dictionaries separately.
  </para>

  <sect2 id="textsearch-configuration-testing">
   <title>Configuration Testing</title>

  <para>
   The function <function>ts_debug</function> allows easy testing of a
   text search configuration.
  </para>

  <indexterm>
   <primary>ts_debug</primary>
  </indexterm>

  <synopsis>
   ts_debug(<optional> <replaceable class="PARAMETER">config</replaceable> <type>regconfig</>, </optional> <replaceable class="PARAMETER">document</replaceable> <type>text</>,
            OUT <replaceable class="PARAMETER">alias</> <type>text</>,
            OUT <replaceable class="PARAMETER">description</> <type>text</>,
            OUT <replaceable class="PARAMETER">token</> <type>text</>,
            OUT <replaceable class="PARAMETER">dictionaries</> <type>regdictionary[]</>,
            OUT <replaceable class="PARAMETER">dictionary</> <type>regdictionary</>,
            OUT <replaceable class="PARAMETER">lexemes</> <type>text[]</>)
            returns setof record
  </synopsis>

  <para>
   <function>ts_debug</> displays information about every token of
   <replaceable class="PARAMETER">document</replaceable> as produced by the
   parser and processed by the configured dictionaries.  It uses the
   configuration specified by <replaceable
   class="PARAMETER">config</replaceable>,
   or <varname>default_text_search_config</varname> if that argument is
   omitted.
  </para>

  <para>
   <function>ts_debug</> returns one row for each token identified in the text
   by the parser.  The columns returned are

    <itemizedlist  spacing="compact" mark="bullet">
     <listitem>
      <para>
       <replaceable>alias</> <type>text</> &mdash; short name of the token type
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>description</> <type>text</> &mdash; description of the
       token type
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>token</> <type>text</> &mdash; text of the token
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>dictionaries</> <type>regdictionary[]</> &mdash; the
       dictionaries selected by the configuration for this token type
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>dictionary</> <type>regdictionary</> &mdash; the dictionary
       that recognized the token, or <literal>NULL</> if none did
      </para>
     </listitem>
     <listitem>
      <para>
       <replaceable>lexemes</> <type>text[]</> &mdash; the lexeme(s) produced
       by the dictionary that recognized the token, or <literal>NULL</> if
       none did; an empty array (<literal>{}</>) means it was recognized as a
       stop word
      </para>
     </listitem>
    </itemizedlist>
  </para>

  <para>
   Here is a simple example:

<programlisting>
SELECT * FROM ts_debug('english','a fat  cat sat on a mat - it ate a fat rats');
   alias   |   description   | token |  dictionaries  |  dictionary  | lexemes 
-----------+-----------------+-------+----------------+--------------+---------
 asciiword | Word, all ASCII | a     | {english_stem} | english_stem | {}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | fat   | {english_stem} | english_stem | {fat}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | cat   | {english_stem} | english_stem | {cat}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | sat   | {english_stem} | english_stem | {sat}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | on    | {english_stem} | english_stem | {}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | a     | {english_stem} | english_stem | {}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | mat   | {english_stem} | english_stem | {mat}
 blank     | Space symbols   |       | {}             |              | 
 blank     | Space symbols   | -     | {}             |              | 
 asciiword | Word, all ASCII | it    | {english_stem} | english_stem | {}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | ate   | {english_stem} | english_stem | {ate}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | a     | {english_stem} | english_stem | {}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | fat   | {english_stem} | english_stem | {fat}
 blank     | Space symbols   |       | {}             |              | 
 asciiword | Word, all ASCII | rats  | {english_stem} | english_stem | {rat}
</programlisting>
  </para>

  <para>
   For a more extensive demonstration, we
   first create a <literal>public.english</literal> configuration and
   Ispell dictionary for the English language:
  </para>

<programlisting>
CREATE TEXT SEARCH CONFIGURATION public.english ( COPY = pg_catalog.english );

CREATE TEXT SEARCH DICTIONARY english_ispell (
    TEMPLATE = ispell,
    DictFile = english,
    AffFile = english,
    StopWords = english
);

ALTER TEXT SEARCH CONFIGURATION public.english
   ALTER MAPPING FOR asciiword WITH english_ispell, english_stem;
</programlisting>

<programlisting>
SELECT * FROM ts_debug('public.english','The Brightest supernovaes');
   alias   |   description   |    token    |         dictionaries          |   dictionary   |   lexemes   
-----------+-----------------+-------------+-------------------------------+----------------+-------------
 asciiword | Word, all ASCII | The         | {english_ispell,english_stem} | english_ispell | {}
 blank     | Space symbols   |             | {}                            |                | 
 asciiword | Word, all ASCII | Brightest   | {english_ispell,english_stem} | english_ispell | {bright}
 blank     | Space symbols   |             | {}                            |                | 
 asciiword | Word, all ASCII | supernovaes | {english_ispell,english_stem} | english_stem   | {supernova}
</programlisting>

  <para>
   In this example, the word <literal>Brightest</> was recognized by the
   parser as an <literal>ASCII word</literal> (alias <literal>asciiword</literal>).
   For this token type the dictionary list is
   <literal>english_ispell</> and
   <literal>english_stem</literal>. The word was recognized by
   <literal>english_ispell</literal>, which reduced it to the noun
   <literal>bright</literal>. The word <literal>supernovaes</literal> is
   unknown to the <literal>english_ispell</literal> dictionary so it
   was passed to the next dictionary, and, fortunately, was recognized (in
   fact, <literal>english_stem</literal> is a Snowball dictionary which
   recognizes everything; that is why it was placed at the end of the
   dictionary list).
  </para>

  <para>
   The word <literal>The</literal> was recognized by the
   <literal>english_ispell</literal> dictionary as a stop word (<xref
   linkend="textsearch-stopwords">) and will not be indexed.
   The spaces are discarded too, since the configuration provides no
   dictionaries at all for them.
  </para>

  <para>
   You can reduce the volume of output by explicitly specifying which columns
   you want to see:

<programlisting>
SELECT alias, token, dictionary, lexemes
FROM ts_debug('public.english','The Brightest supernovaes');
   alias   |    token    |   dictionary   |   lexemes   
-----------+-------------+----------------+-------------
 asciiword | The         | english_ispell | {}
 blank     |             |                | 
 asciiword | Brightest   | english_ispell | {bright}
 blank     |             |                | 
 asciiword | supernovaes | english_stem   | {supernova}
</programlisting>
  </para>

  </sect2>

  <sect2 id="textsearch-parser-testing">
   <title>Parser Testing</title>

  <para>
   The following functions allow direct testing of a text search parser.
  </para>

  <indexterm>
   <primary>ts_parse</primary>
  </indexterm>

  <synopsis>
   ts_parse(<replaceable class="PARAMETER">parser_name</replaceable> <type>text</>, <replaceable class="PARAMETER">document</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">token</> <type>text</>) returns <type>setof record</>
   ts_parse(<replaceable class="PARAMETER">parser_oid</replaceable> <type>oid</>, <replaceable class="PARAMETER">document</replaceable> <type>text</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">token</> <type>text</>) returns <type>setof record</>
  </synopsis>

  <para>
   <function>ts_parse</> parses the given <replaceable>document</replaceable>
   and returns a series of records, one for each token produced by
   parsing. Each record includes a <varname>tokid</varname> showing the
   assigned token type and a <varname>token</varname> which is the text of the
   token.  For example:

<programlisting>
SELECT * FROM ts_parse('default', '123 - a number');
 tokid | token
-------+--------
    22 | 123
    12 |
    12 | -
     1 | a
    12 |
     1 | number
</programlisting>
  </para>

  <indexterm>
   <primary>ts_token_type</primary>
  </indexterm>

  <synopsis>
   ts_token_type(<replaceable class="PARAMETER">parser_name</> <type>text</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">alias</> <type>text</>, OUT <replaceable class="PARAMETER">description</> <type>text</>) returns <type>setof record</>
   ts_token_type(<replaceable class="PARAMETER">parser_oid</> <type>oid</>, OUT <replaceable class="PARAMETER">tokid</> <type>integer</>, OUT <replaceable class="PARAMETER">alias</> <type>text</>, OUT <replaceable class="PARAMETER">description</> <type>text</>) returns <type>setof record</>
  </synopsis>

  <para>
   <function>ts_token_type</> returns a table which describes each type of
   token the specified parser can recognize.  For each token type, the table
   gives the integer <varname>tokid</varname> that the parser uses to label a
   token of that type, the <varname>alias</varname> that names the token type
   in configuration commands, and a short <varname>description</varname>.  For
   example:

<programlisting>
SELECT * FROM ts_token_type('default');
 tokid |      alias      |               description                
-------+-----------------+------------------------------------------
     1 | asciiword       | Word, all ASCII
     2 | word            | Word, all letters
     3 | numword         | Word, letters and digits
     4 | email           | Email address
     5 | url             | URL
     6 | host            | Host
     7 | sfloat          | Scientific notation
     8 | version         | Version number
     9 | hword_numpart   | Hyphenated word part, letters and digits
    10 | hword_part      | Hyphenated word part, all letters
    11 | hword_asciipart | Hyphenated word part, all ASCII
    12 | blank           | Space symbols
    13 | tag             | XML tag
    14 | protocol        | Protocol head
    15 | numhword        | Hyphenated word, letters and digits
    16 | asciihword      | Hyphenated word, all ASCII
    17 | hword           | Hyphenated word, all letters
    18 | url_path        | URL path
    19 | file            | File or path name
    20 | float           | Decimal notation
    21 | int             | Signed integer
    22 | uint            | Unsigned integer
    23 | entity          | XML entity
</programlisting>
   </para>

  </sect2>

  <sect2 id="textsearch-dictionary-testing">
   <title>Dictionary Testing</title>

   <para>
    The <function>ts_lexize</> function facilitates dictionary testing.
   </para>

   <indexterm>
    <primary>ts_lexize</primary>
   </indexterm>

   <synopsis>
    ts_lexize(<replaceable class="PARAMETER">dict</replaceable> <type>regdictionary</>, <replaceable class="PARAMETER">token</replaceable> <type>text</>) returns <type>text[]</>
   </synopsis>

   <para>
    <function>ts_lexize</> returns an array of lexemes if the input
    <replaceable>token</replaceable> is known to the dictionary,
    or an empty array if the token
    is known to the dictionary but it is a stop word, or
    <literal>NULL</literal> if it is an unknown word.
   </para>

   <para>
    Examples:

<programlisting>
SELECT ts_lexize('english_stem', 'stars');
 ts_lexize
-----------
 {star}

SELECT ts_lexize('english_stem', 'a');
 ts_lexize
-----------
 {}
</programlisting>
   </para>

   <note>
    <para>
     The <function>ts_lexize</function> function expects a single
     <emphasis>token</emphasis>, not text. Here is a case
     where this can be confusing:

<programlisting>
SELECT ts_lexize('thesaurus_astro','supernovae stars') is null;
 ?column?
----------
 t
</programlisting>

     The thesaurus dictionary <literal>thesaurus_astro</literal> does know the
     phrase <literal>supernovae stars</literal>, but <function>ts_lexize</>
     fails since it does not parse the input text but treats it as a single
     token. Use <function>plainto_tsquery</> or <function>to_tsvector</> to
     test thesaurus dictionaries, for example:

<programlisting>
SELECT plainto_tsquery('supernovae stars');
 plainto_tsquery
-----------------
 'sn'
</programlisting>
    </para>
   </note>

  </sect2>

 </sect1>

 <sect1 id="textsearch-indexes">
  <title>GiST and GIN Index Types</title>

  <indexterm zone="textsearch-indexes">
   <primary>text search</primary>
   <secondary>indexes</secondary>
  </indexterm>

  <para>
   There are two kinds of indexes that can be used to speed up full text
   searches.
   Note that indexes are not mandatory for full text searching, but in
   cases where a column is searched on a regular basis, an index will
   usually be desirable.

   <variablelist>

    <varlistentry>

     <indexterm zone="textsearch-indexes">
      <primary>index</primary>
      <secondary>GiST</secondary>
      <tertiary>text search</tertiary>
     </indexterm>

     <term>
      <synopsis>
       CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING gist(<replaceable>column</replaceable>);
      </synopsis>
     </term>

     <listitem>
      <para>
       Creates a GiST (Generalized Search Tree)-based index.
       The <replaceable>column</replaceable> can be of <type>tsvector</> or
       <type>tsquery</> type.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>

     <indexterm zone="textsearch-indexes">
      <primary>index</primary>
      <secondary>GIN</secondary>
      <tertiary>text search</tertiary>
     </indexterm>

     <term>
      <synopsis>
       CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING gin(<replaceable>column</replaceable>);
      </synopsis>
     </term>

     <listitem>
      <para>
       Creates a GIN (Generalized Inverted Index)-based index.
       The <replaceable>column</replaceable> must be of <type>tsvector</> type.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

  <para>
   There are substantial performance differences between the two index types,
   so it is important to understand which to use.
  </para>

  <para>
   A GiST index is <firstterm>lossy</firstterm>, meaning that the index
   may produce false matches, and it is necessary
   to check the actual table row to eliminate such false matches.
   <productname>PostgreSQL</productname> does this automatically; for
   example, in the query plan below, the <literal>Filter:</literal>
   line indicates the index output will be rechecked:

<programlisting>
EXPLAIN SELECT * FROM apod WHERE textsearch @@ to_tsquery('supernovae');
                               QUERY PLAN
-------------------------------------------------------------------------
 Index Scan using textsearch_gidx on apod  (cost=0.00..12.29 rows=2 width=1469)
   Index Cond: (textsearch @@ '''supernova'''::tsquery)
   Filter: (textsearch @@ '''supernova'''::tsquery)
</programlisting>

   GiST indexes are lossy because each document is represented in the
   index by a fixed-length signature. The signature is generated by hashing
   each word into a random bit in an n-bit string, with all these bits OR-ed
   together to produce an n-bit document signature.  When two words hash to
   the same bit position there will be a false match.  If all words in
   the query have matches (real or false) then the table row must be
   retrieved to see if the match is correct.
  </para>

  <para>
   Lossiness causes performance degradation due to useless fetches of table
   records that turn out to be false matches.  Since random access to table
   records is slow, this limits the usefulness of GiST indexes.  The
   likelihood of false matches depends on several factors, in particular the
   number of unique words, so using dictionaries to reduce this number is
   recommended.
  </para>

  <para>
   GIN indexes are not lossy but their performance depends logarithmically on
   the number of unique words.
  </para>

  <para>
   Actually, GIN indexes store only the words (lexemes) of <type>tsvector</>
   values, and not their weight labels.  Thus, while a GIN index can be
   considered non-lossy for a query that does not specify weights, it is
   lossy for one that does.  Thus a table row recheck is needed when using
   a query that involves weights.  Unfortunately, in the current design of
   <productname>PostgreSQL</>, whether a recheck is needed is a static
   property of a particular operator, and not something that can be enabled
   or disabled on-the-fly depending on the values given to the operator.
   To deal with this situation without imposing the overhead of rechecks
   on queries that do not need them, the following approach has been
   adopted:
  </para>

  <itemizedlist  spacing="compact" mark="bullet">
   <listitem>
    <para>
     The standard text match operator <literal>@@</> is marked as non-lossy
     for GIN indexes.
    </para>
   </listitem>

   <listitem>
    <para>
     An additional match operator <literal>@@@</> is provided, and marked
     as lossy for GIN indexes.  This operator behaves exactly like
     <literal>@@</> otherwise.
    </para>
   </listitem>

   <listitem>
    <para>
     When a GIN index search is initiated with the <literal>@@</> operator,
     the index support code will throw an error if the query specifies any
     weights.  This protects against giving wrong answers due to failure
     to recheck the weights.
    </para>
   </listitem>
  </itemizedlist>

  <para>
   In short, you must use <literal>@@@</> rather than <literal>@@</> to
   perform GIN index searches on queries that involve weight restrictions.
   For queries that do not have weight restrictions, either operator will
   work, but <literal>@@</> will be faster.
   This awkwardness will probably be addressed in a future release of
   <productname>PostgreSQL</>.
  </para>

  <para>
   In choosing which index type to use, GiST or GIN, consider these
   performance differences:

   <itemizedlist  spacing="compact" mark="bullet">
    <listitem>
     <para>
      GIN index lookups are about three times faster than GiST
     </para>
    </listitem>
    <listitem>
     <para>
      GIN indexes take about three times longer to build than GiST
     </para>
    </listitem>
    <listitem>
     <para>
      GIN indexes are about ten times slower to update than GiST
     </para>
    </listitem>
    <listitem>
     <para>
      GIN indexes are two-to-three times larger than GiST
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   As a rule of thumb, <acronym>GIN</acronym> indexes are best for static data
   because lookups are faster.  For dynamic data, GiST indexes are
   faster to update.  Specifically, <acronym>GiST</acronym> indexes are very
   good for dynamic data and fast if the number of unique words (lexemes) is
   under 100,000, while <acronym>GIN</acronym> indexes will handle 100,000+
   lexemes better but are slower to update.
  </para>

  <para>
   Note that <acronym>GIN</acronym> index build time can often be improved
   by increasing <xref linkend="guc-maintenance-work-mem">, while
   <acronym>GiST</acronym> index build time is not sensitive to that
   parameter.
  </para>

  <para>
   Partitioning of big collections and the proper use of GiST and GIN indexes
   allows the implementation of very fast searches with online update.
   Partitioning can be done at the database level using table inheritance
   and <varname>constraint_exclusion</>, or by distributing documents over
   servers and collecting search results using the <filename>contrib/dblink</>
   extension module. The latter is possible because ranking functions use
   only local information.
  </para>

 </sect1>

 <sect1 id="textsearch-psql">
  <title><application>psql</> Support</title>

  <para>
   Information about text search configuration objects can be obtained
   in <application>psql</application> using a set of commands:
   <synopsis>
   \dF{d,p,t}<optional>+</optional> <optional>PATTERN</optional>
   </synopsis>
   An optional <literal>+</literal> produces more details.
  </para>

  <para>
   The optional parameter <literal>PATTERN</literal> should be the name of
   a text search object, optionally schema-qualified.  If
   <literal>PATTERN</literal> is omitted then information about all
   visible objects will be displayed.  <literal>PATTERN</literal> can be a
   regular expression and can provide <emphasis>separate</emphasis> patterns
   for the schema and object names.  The following examples illustrate this:

<programlisting>
=&gt; \dF *fulltext*
       List of text search configurations
 Schema |  Name        | Description
--------+--------------+-------------
 public | fulltext_cfg |
</programlisting>

<programlisting>
=&gt; \dF *.fulltext*
       List of text search configurations
 Schema   |  Name        | Description
----------+----------------------------
 fulltext | fulltext_cfg |
 public   | fulltext_cfg |
</programlisting>

   The available commands are:
  </para>

  <variablelist>

   <varlistentry>
    <term><synopsis>\dF<optional>+</optional> <optional>PATTERN</optional></synopsis></term>

    <listitem>
     <para>
      List text search configurations (add <literal>+</> for more detail).
     </para>

     <para>

<programlisting>
=&gt; \dF russian
            List of text search configurations
   Schema   |  Name   |            Description             
------------+---------+------------------------------------
 pg_catalog | russian | configuration for russian language

=&gt; \dF+ russian
Text search configuration "pg_catalog.russian"
Parser: "pg_catalog.default"
      Token      | Dictionaries 
-----------------+--------------
 asciihword      | english_stem
 asciiword       | english_stem
 email           | simple
 file            | simple
 float           | simple
 host            | simple
 hword           | russian_stem
 hword_asciipart | english_stem
 hword_numpart   | simple
 hword_part      | russian_stem
 int             | simple
 numhword        | simple
 numword         | simple
 sfloat          | simple
 uint            | simple
 url             | simple
 url_path        | simple
 version         | simple
 word            | russian_stem
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><synopsis>\dFd<optional>+</optional> <optional>PATTERN</optional></synopsis></term>
    <listitem>
     <para>
      List text search dictionaries (add <literal>+</> for more detail).
     </para>

     <para>
<programlisting>
=&gt; \dFd
                            List of text search dictionaries
   Schema   |      Name       |                        Description                        
------------+-----------------+-----------------------------------------------------------
 pg_catalog | danish_stem     | snowball stemmer for danish language
 pg_catalog | dutch_stem      | snowball stemmer for dutch language
 pg_catalog | english_stem    | snowball stemmer for english language
 pg_catalog | finnish_stem    | snowball stemmer for finnish language
 pg_catalog | french_stem     | snowball stemmer for french language
 pg_catalog | german_stem     | snowball stemmer for german language
 pg_catalog | hungarian_stem  | snowball stemmer for hungarian language
 pg_catalog | italian_stem    | snowball stemmer for italian language
 pg_catalog | norwegian_stem  | snowball stemmer for norwegian language
 pg_catalog | portuguese_stem | snowball stemmer for portuguese language
 pg_catalog | romanian_stem   | snowball stemmer for romanian language
 pg_catalog | russian_stem    | snowball stemmer for russian language
 pg_catalog | simple          | simple dictionary: just lower case and check for stopword
 pg_catalog | spanish_stem    | snowball stemmer for spanish language
 pg_catalog | swedish_stem    | snowball stemmer for swedish language
 pg_catalog | turkish_stem    | snowball stemmer for turkish language
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>

   <term><synopsis>\dFp<optional>+</optional> <optional>PATTERN</optional></synopsis></term>
    <listitem>
     <para>
      List text search parsers (add <literal>+</> for more detail).
     </para>

     <para>
<programlisting>
=&gt; \dFp
        List of text search parsers
   Schema   |  Name   |     Description     
------------+---------+---------------------
 pg_catalog | default | default word parser
=&gt; \dFp+
    Text search parser "pg_catalog.default"
     Method      |    Function    | Description 
-----------------+----------------+-------------
 Start parse     | prsd_start     | 
 Get next token  | prsd_nexttoken | 
 End parse       | prsd_end       | 
 Get headline    | prsd_headline  | 
 Get token types | prsd_lextype   | 

        Token types for parser "pg_catalog.default"
   Token name    |               Description                
-----------------+------------------------------------------
 asciihword      | Hyphenated word, all ASCII
 asciiword       | Word, all ASCII
 blank           | Space symbols
 email           | Email address
 entity          | XML entity
 file            | File or path name
 float           | Decimal notation
 host            | Host
 hword           | Hyphenated word, all letters
 hword_asciipart | Hyphenated word part, all ASCII
 hword_numpart   | Hyphenated word part, letters and digits
 hword_part      | Hyphenated word part, all letters
 int             | Signed integer
 numhword        | Hyphenated word, letters and digits
 numword         | Word, letters and digits
 protocol        | Protocol head
 sfloat          | Scientific notation
 tag             | XML tag
 uint            | Unsigned integer
 url             | URL
 url_path        | URL path
 version         | Version number
 word            | Word, all letters
(23 rows)
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>

   <term><synopsis>\dFt<optional>+</optional> <optional>PATTERN</optional></synopsis></term>
    <listitem>
     <para>
      List text search templates (add <literal>+</> for more detail).
     </para>

     <para>
<programlisting>
=&gt; \dFt
                           List of text search templates
   Schema   |   Name    |                        Description                        
------------+-----------+-----------------------------------------------------------
 pg_catalog | ispell    | ispell dictionary
 pg_catalog | simple    | simple dictionary: just lower case and check for stopword
 pg_catalog | snowball  | snowball stemmer
 pg_catalog | synonym   | synonym dictionary: replace word by its synonym
 pg_catalog | thesaurus | thesaurus dictionary: phrase by phrase substitution
</programlisting>
     </para>
    </listitem>
   </varlistentry>

  </variablelist>

 </sect1>

 <sect1 id="textsearch-limitations">
  <title>Limitations</title>

  <para>
   The current limitations of <productname>PostgreSQL</productname>'s
   text search features are:
   <itemizedlist  spacing="compact" mark="bullet">
    <listitem>
     <para>The length of each lexeme must be less than 2K bytes</para>
    </listitem>
    <listitem>
     <para>The length of a <type>tsvector</type> (lexemes + positions) must be
     less than 1 megabyte</para>
    </listitem>
    <listitem>
     <!-- TODO: number of lexemes in what?  This is unclear -->
     <para>The number of lexemes must be less than
     2<superscript>64</superscript></para>
    </listitem>
    <listitem>
     <para>Position values in <type>tsvector</> must be greater than 0 and
     no more than 16,383</para>
    </listitem>
    <listitem>
     <para>No more than 256 positions per lexeme</para>
    </listitem>
    <listitem>
     <para>The number of nodes (lexemes + operators) in a <type>tsquery</type>
     must be less than 32,768</para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   For comparison, the <productname>PostgreSQL</productname> 8.1 documentation
   contained 10,441 unique words, a total of 335,420 words, and the most
   frequent word <quote>postgresql</> was mentioned 6,127 times in 655
   documents.
  </para>

   <!-- TODO we need to put a date on these numbers? -->
  <para>
   Another example &mdash; the <productname>PostgreSQL</productname> mailing
   list archives contained 910,989 unique words with 57,491,343 lexemes in
   461,020 messages.
  </para>

 </sect1>

 <sect1 id="textsearch-migration">
  <title>Migration from Pre-8.3 Text Search</title>

  <para>
   Applications that used the <filename>contrib/tsearch2</> add-on module
   for text searching will need some adjustments to work with the
   built-in features:
  </para>

  <itemizedlist>
   <listitem>
    <para>
     Some functions have been renamed or had small adjustments in their
     argument lists, and all of them are now in the <literal>pg_catalog</>
     schema, whereas in a previous installation they would have been in
     <literal>public</> or another non-system schema.  There is a new
     version of <filename>contrib/tsearch2</> (see <xref linkend="tsearch2">)
     that provides a compatibility layer to solve most problems in this
     area.
    </para>
   </listitem>

   <listitem>
    <para>
     The old <filename>contrib/tsearch2</> functions and other objects
     <emphasis>must</> be suppressed when loading <application>pg_dump</>
     output from a pre-8.3 database.  While many of them won't load anyway,
     a few will and then cause problems.  One simple way to deal with this
     is to load the new <filename>contrib/tsearch2</> module before restoring
     the dump; then it will block the old objects from being loaded.
    </para>
   </listitem>

   <listitem>
    <para>
     Text search configuration setup is completely different now.
     Instead of manually inserting rows into configuration tables,
     search is configured through the specialized SQL commands shown
     earlier in this chapter.  There is not currently any automated
     support for converting an existing custom configuration for 8.3;
     you're on your own here.
    </para>
   </listitem>

   <listitem>
    <para>
     Most types of dictionaries rely on some outside-the-database
     configuration files.  These are largely compatible with pre-8.3
     usage, but note the following differences:

     <itemizedlist  spacing="compact" mark="bullet">
      <listitem>
       <para>
        Configuration files now must be placed in a single specified
        directory (<filename>$SHAREDIR/tsearch_data</>), and must have
        a specific extension depending on the type of file, as noted
        previously in the descriptions of the various dictionary types.
        This restriction was added to forestall security problems.
       </para>
      </listitem>

      <listitem>
       <para>
        Configuration files must be encoded in UTF-8 encoding,
        regardless of what database encoding is used.
       </para>
      </listitem>

      <listitem>
       <para>
        In thesaurus configuration files, stop words must be marked with
        <literal>?</>.
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </listitem>

  </itemizedlist>

 </sect1>

</chapter>
