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| author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:17:33 +0000 |
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| committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-05-04 12:17:33 +0000 |
| commit | 5e45211a64149b3c659b90ff2de6fa982a5a93ed (patch) | |
| tree | 739caf8c461053357daa9f162bef34516c7bf452 /doc/src/sgml/html/textsearch-features.html | |
| parent | Initial commit. (diff) | |
| download | postgresql-15-5e45211a64149b3c659b90ff2de6fa982a5a93ed.tar.xz postgresql-15-5e45211a64149b3c659b90ff2de6fa982a5a93ed.zip | |
Adding upstream version 15.5.upstream/15.5
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'doc/src/sgml/html/textsearch-features.html')
| -rw-r--r-- | doc/src/sgml/html/textsearch-features.html | 392 |
1 files changed, 392 insertions, 0 deletions
diff --git a/doc/src/sgml/html/textsearch-features.html b/doc/src/sgml/html/textsearch-features.html new file mode 100644 index 0000000..9e5ae4f --- /dev/null +++ b/doc/src/sgml/html/textsearch-features.html @@ -0,0 +1,392 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>12.4. Additional Features</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="textsearch-controls.html" title="12.3. Controlling Text Search" /><link rel="next" href="textsearch-parsers.html" title="12.5. Parsers" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">12.4. Additional Features</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="textsearch-controls.html" title="12.3. Controlling Text Search">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="textsearch.html" title="Chapter 12. Full Text Search">Up</a></td><th width="60%" align="center">Chapter 12. Full Text Search</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="textsearch-parsers.html" title="12.5. Parsers">Next</a></td></tr></table><hr /></div><div class="sect1" id="TEXTSEARCH-FEATURES"><div class="titlepage"><div><div><h2 class="title" style="clear: both">12.4. Additional Features</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="textsearch-features.html#TEXTSEARCH-MANIPULATE-TSVECTOR">12.4.1. Manipulating Documents</a></span></dt><dt><span class="sect2"><a href="textsearch-features.html#TEXTSEARCH-MANIPULATE-TSQUERY">12.4.2. Manipulating Queries</a></span></dt><dt><span class="sect2"><a href="textsearch-features.html#TEXTSEARCH-UPDATE-TRIGGERS">12.4.3. Triggers for Automatic Updates</a></span></dt><dt><span class="sect2"><a href="textsearch-features.html#TEXTSEARCH-STATISTICS">12.4.4. Gathering Document Statistics</a></span></dt></dl></div><p> + This section describes additional functions and operators that are + useful in connection with text search. + </p><div class="sect2" id="TEXTSEARCH-MANIPULATE-TSVECTOR"><div class="titlepage"><div><div><h3 class="title">12.4.1. Manipulating Documents</h3></div></div></div><p> + <a class="xref" href="textsearch-controls.html#TEXTSEARCH-PARSING-DOCUMENTS" title="12.3.1. Parsing Documents">Section 12.3.1</a> showed how raw textual + documents can be converted into <code class="type">tsvector</code> values. + <span class="productname">PostgreSQL</span> also provides functions and + operators that can be used to manipulate documents that are already + in <code class="type">tsvector</code> form. + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"> + <a id="id-1.5.11.7.3.3.1.1.1" class="indexterm"></a> + + <code class="literal"><code class="type">tsvector</code> || <code class="type">tsvector</code></code> + </span></dt><dd><p> + The <code class="type">tsvector</code> 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 <code class="function">to_tsvector</code> + 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.) + </p><p> + One advantage of using concatenation in the vector form, rather than + concatenating text before applying <code class="function">to_tsvector</code>, is that + you can use different configurations to parse different sections + of the document. Also, because the <code class="function">setweight</code> function + marks all lexemes of the given vector the same way, it is necessary + to parse the text and do <code class="function">setweight</code> before concatenating + if you want to label different parts of the document with different + weights. + </p></dd><dt><span class="term"> + <a id="id-1.5.11.7.3.3.2.1.1" class="indexterm"></a> + + <code class="literal">setweight(<em class="replaceable"><code>vector</code></em> <code class="type">tsvector</code>, <em class="replaceable"><code>weight</code></em> <code class="type">"char"</code>) returns <code class="type">tsvector</code></code> + </span></dt><dd><p> + <code class="function">setweight</code> returns a copy of the input vector in which every + position has been labeled with the given <em class="replaceable"><code>weight</code></em>, either + <code class="literal">A</code>, <code class="literal">B</code>, <code class="literal">C</code>, or + <code class="literal">D</code>. (<code class="literal">D</code> 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. + </p><p> + Note that weight labels apply to <span class="emphasis"><em>positions</em></span>, not + <span class="emphasis"><em>lexemes</em></span>. If the input vector has been stripped of + positions then <code class="function">setweight</code> does nothing. + </p></dd><dt><span class="term"> + <a id="id-1.5.11.7.3.3.3.1.1" class="indexterm"></a> + + <code class="literal">length(<em class="replaceable"><code>vector</code></em> <code class="type">tsvector</code>) returns <code class="type">integer</code></code> + </span></dt><dd><p> + Returns the number of lexemes stored in the vector. + </p></dd><dt><span class="term"> + <a id="id-1.5.11.7.3.3.4.1.1" class="indexterm"></a> + + <code class="literal">strip(<em class="replaceable"><code>vector</code></em> <code class="type">tsvector</code>) returns <code class="type">tsvector</code></code> + </span></dt><dd><p> + Returns a vector that lists the same lexemes as the given vector, but + lacks any position or weight information. The result is usually much + smaller than an unstripped vector, but it is also less useful. + Relevance ranking does not work as well on stripped vectors as + unstripped ones. Also, + the <code class="literal"><-></code> (FOLLOWED BY) <code class="type">tsquery</code> operator + will never match stripped input, since it cannot determine the + distance between lexeme occurrences. + </p></dd></dl></div><p> + A full list of <code class="type">tsvector</code>-related functions is available + in <a class="xref" href="functions-textsearch.html#TEXTSEARCH-FUNCTIONS-TABLE" title="Table 9.43. Text Search Functions">Table 9.43</a>. + </p></div><div class="sect2" id="TEXTSEARCH-MANIPULATE-TSQUERY"><div class="titlepage"><div><div><h3 class="title">12.4.2. Manipulating Queries</h3></div></div></div><p> + <a class="xref" href="textsearch-controls.html#TEXTSEARCH-PARSING-QUERIES" title="12.3.2. Parsing Queries">Section 12.3.2</a> showed how raw textual + queries can be converted into <code class="type">tsquery</code> values. + <span class="productname">PostgreSQL</span> also provides functions and + operators that can be used to manipulate queries that are already + in <code class="type">tsquery</code> form. + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"> + <code class="literal"><code class="type">tsquery</code> && <code class="type">tsquery</code></code> + </span></dt><dd><p> + Returns the AND-combination of the two given queries. + </p></dd><dt><span class="term"> + <code class="literal"><code class="type">tsquery</code> || <code class="type">tsquery</code></code> + </span></dt><dd><p> + Returns the OR-combination of the two given queries. + </p></dd><dt><span class="term"> + <code class="literal">!! <code class="type">tsquery</code></code> + </span></dt><dd><p> + Returns the negation (NOT) of the given query. + </p></dd><dt><span class="term"> + <code class="literal"><code class="type">tsquery</code> <-> <code class="type">tsquery</code></code> + </span></dt><dd><p> + Returns a query that searches for a match to the first given query + immediately followed by a match to the second given query, using + the <code class="literal"><-></code> (FOLLOWED BY) + <code class="type">tsquery</code> operator. For example: + +</p><pre class="screen"> +SELECT to_tsquery('fat') <-> to_tsquery('cat | rat'); + ?column? +---------------------------- + 'fat' <-> ( 'cat' | 'rat' ) +</pre><p> + </p></dd><dt><span class="term"> + <a id="id-1.5.11.7.4.3.5.1.1" class="indexterm"></a> + + <code class="literal">tsquery_phrase(<em class="replaceable"><code>query1</code></em> <code class="type">tsquery</code>, <em class="replaceable"><code>query2</code></em> <code class="type">tsquery</code> [, <em class="replaceable"><code>distance</code></em> <code class="type">integer</code> ]) returns <code class="type">tsquery</code></code> + </span></dt><dd><p> + Returns a query that searches for a match to the first given query + followed by a match to the second given query at a distance of exactly + <em class="replaceable"><code>distance</code></em> lexemes, using + the <code class="literal"><<em class="replaceable"><code>N</code></em>></code> + <code class="type">tsquery</code> operator. For example: + +</p><pre class="screen"> +SELECT tsquery_phrase(to_tsquery('fat'), to_tsquery('cat'), 10); + tsquery_phrase +------------------ + 'fat' <10> 'cat' +</pre><p> + </p></dd><dt><span class="term"> + <a id="id-1.5.11.7.4.3.6.1.1" class="indexterm"></a> + + <code class="literal">numnode(<em class="replaceable"><code>query</code></em> <code class="type">tsquery</code>) returns <code class="type">integer</code></code> + </span></dt><dd><p> + Returns the number of nodes (lexemes plus operators) in a + <code class="type">tsquery</code>. This function is useful + to determine if the <em class="replaceable"><code>query</code></em> is meaningful + (returns > 0), or contains only stop words (returns 0). + Examples: + +</p><pre class="screen"> +SELECT numnode(plainto_tsquery('the any')); +NOTICE: query contains only stopword(s) or doesn't contain lexeme(s), ignored + numnode +--------- + 0 + +SELECT numnode('foo & bar'::tsquery); + numnode +--------- + 3 +</pre><p> + </p></dd><dt><span class="term"> + <a id="id-1.5.11.7.4.3.7.1.1" class="indexterm"></a> + + <code class="literal">querytree(<em class="replaceable"><code>query</code></em> <code class="type">tsquery</code>) returns <code class="type">text</code></code> + </span></dt><dd><p> + Returns the portion of a <code class="type">tsquery</code> 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: + +</p><pre class="screen"> +SELECT querytree(to_tsquery('defined')); + querytree +----------- + 'defin' + +SELECT querytree(to_tsquery('!defined')); + querytree +----------- + T +</pre><p> + </p></dd></dl></div><div class="sect3" id="TEXTSEARCH-QUERY-REWRITING"><div class="titlepage"><div><div><h4 class="title">12.4.2.1. Query Rewriting</h4></div></div></div><a id="id-1.5.11.7.4.4.2" class="indexterm"></a><p> + The <code class="function">ts_rewrite</code> family of functions search a + given <code class="type">tsquery</code> for occurrences of a target + subquery, and replace each occurrence with a + substitute subquery. In essence this operation is a + <code class="type">tsquery</code>-specific version of substring replacement. + A target and substitute combination can be + thought of as a <em class="firstterm">query rewrite rule</em>. A collection + of such rewrite rules can be a powerful search aid. + For example, you can expand the search using synonyms + (e.g., <code class="literal">new york</code>, <code class="literal">big apple</code>, <code class="literal">nyc</code>, + <code class="literal">gotham</code>) or narrow the search to direct the user to some hot + topic. There is some overlap in functionality between this feature + and thesaurus dictionaries (<a class="xref" href="textsearch-dictionaries.html#TEXTSEARCH-THESAURUS" title="12.6.4. Thesaurus Dictionary">Section 12.6.4</a>). + However, you can modify a set of rewrite rules on-the-fly without + reindexing, whereas updating a thesaurus requires reindexing to be + effective. + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"> + <code class="literal">ts_rewrite (<em class="replaceable"><code>query</code></em> <code class="type">tsquery</code>, <em class="replaceable"><code>target</code></em> <code class="type">tsquery</code>, <em class="replaceable"><code>substitute</code></em> <code class="type">tsquery</code>) returns <code class="type">tsquery</code></code> + </span></dt><dd><p> + This form of <code class="function">ts_rewrite</code> simply applies a single + rewrite rule: <em class="replaceable"><code>target</code></em> + is replaced by <em class="replaceable"><code>substitute</code></em> + wherever it appears in <em class="replaceable"><code>query</code></em>. For example: + +</p><pre class="screen"> +SELECT ts_rewrite('a & b'::tsquery, 'a'::tsquery, 'c'::tsquery); + ts_rewrite +------------ + 'b' & 'c' +</pre><p> + </p></dd><dt><span class="term"> + <code class="literal">ts_rewrite (<em class="replaceable"><code>query</code></em> <code class="type">tsquery</code>, <em class="replaceable"><code>select</code></em> <code class="type">text</code>) returns <code class="type">tsquery</code></code> + </span></dt><dd><p> + This form of <code class="function">ts_rewrite</code> accepts a starting + <em class="replaceable"><code>query</code></em> and an SQL <em class="replaceable"><code>select</code></em> command, which + is given as a text string. The <em class="replaceable"><code>select</code></em> must yield two + columns of <code class="type">tsquery</code> type. For each row of the + <em class="replaceable"><code>select</code></em> result, occurrences of the first column value + (the target) are replaced by the second column value (the substitute) + within the current <em class="replaceable"><code>query</code></em> value. For example: + +</p><pre class="screen"> +CREATE TABLE aliases (t tsquery PRIMARY KEY, s tsquery); +INSERT INTO aliases VALUES('a', 'c'); + +SELECT ts_rewrite('a & b'::tsquery, 'SELECT t,s FROM aliases'); + ts_rewrite +------------ + 'b' & 'c' +</pre><p> + </p><p> + 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 <code class="literal">ORDER BY</code> some ordering key. + </p></dd></dl></div><p> + Let's consider a real-life astronomical example. We'll expand query + <code class="literal">supernovae</code> using table-driven rewriting rules: + +</p><pre class="screen"> +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 & crab'), 'SELECT * FROM aliases'); + ts_rewrite +--------------------------------- + 'crab' & ( 'supernova' | 'sn' ) +</pre><p> + + We can change the rewriting rules just by updating the table: + +</p><pre class="screen"> +UPDATE aliases +SET s = to_tsquery('supernovae|sn & !nebulae') +WHERE t = to_tsquery('supernovae'); + +SELECT ts_rewrite(to_tsquery('supernovae & crab'), 'SELECT * FROM aliases'); + ts_rewrite +--------------------------------------------- + 'crab' & ( 'supernova' | 'sn' & !'nebula' ) +</pre><p> + </p><p> + Rewriting can be slow when there are many rewriting rules, since it + checks every rule for a possible match. To filter out obvious non-candidate + rules we can use the containment operators for the <code class="type">tsquery</code> + type. In the example below, we select only those rules which might match + the original query: + +</p><pre class="screen"> +SELECT ts_rewrite('a & b'::tsquery, + 'SELECT t,s FROM aliases WHERE ''a & b''::tsquery @> t'); + ts_rewrite +------------ + 'b' & 'c' +</pre><p> + </p></div></div><div class="sect2" id="TEXTSEARCH-UPDATE-TRIGGERS"><div class="titlepage"><div><div><h3 class="title">12.4.3. Triggers for Automatic Updates</h3></div></div></div><a id="id-1.5.11.7.5.2" class="indexterm"></a><div class="note"><h3 class="title">Note</h3><p> + The method described in this section has been obsoleted by the use of + stored generated columns, as described in <a class="xref" href="textsearch-tables.html#TEXTSEARCH-TABLES-INDEX" title="12.2.2. Creating Indexes">Section 12.2.2</a>. + </p></div><p> + When using a separate column to store the <code class="type">tsvector</code> representation + of your documents, it is necessary to create a trigger to update the + <code class="type">tsvector</code> column when the document content columns change. + Two built-in trigger functions are available for this, or you can write + your own. + </p><pre class="synopsis"> +tsvector_update_trigger(<em class="replaceable"><code>tsvector_column_name</code></em>, <em class="replaceable"><code>config_name</code></em>, <em class="replaceable"><code>text_column_name</code></em> [<span class="optional">, ... </span>]) +tsvector_update_trigger_column(<em class="replaceable"><code>tsvector_column_name</code></em>, <em class="replaceable"><code>config_column_name</code></em>, <em class="replaceable"><code>text_column_name</code></em> [<span class="optional">, ... </span>]) +</pre><p> + These trigger functions automatically compute a <code class="type">tsvector</code> + column from one or more textual columns, under the control of + parameters specified in the <code class="command">CREATE TRIGGER</code> command. + An example of their use is: + +</p><pre class="screen"> +CREATE TABLE messages ( + title text, + body text, + tsv tsvector +); + +CREATE TRIGGER tsvectorupdate BEFORE INSERT OR UPDATE +ON messages FOR EACH ROW EXECUTE FUNCTION +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 & body'); + title | body +------------+----------------------- + title here | the body text is here +</pre><p> + + Having created this trigger, any change in <code class="structfield">title</code> or + <code class="structfield">body</code> will automatically be reflected into + <code class="structfield">tsv</code>, without the application having to worry about it. + </p><p> + The first trigger argument must be the name of the <code class="type">tsvector</code> + column to be updated. The second argument specifies the text search + configuration to be used to perform the conversion. For + <code class="function">tsvector_update_trigger</code>, 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 <code class="varname">search_path</code>. For + <code class="function">tsvector_update_trigger_column</code>, the second trigger argument + is the name of another table column, which must be of type + <code class="type">regconfig</code>. This allows a per-row selection of configuration + to be made. The remaining argument(s) are the names of textual columns + (of type <code class="type">text</code>, <code class="type">varchar</code>, or <code class="type">char</code>). These + will be included in the document in the order given. NULL values will + be skipped (but the other columns will still be indexed). + </p><p> + A limitation of these built-in triggers is that they treat all the + input columns alike. To process columns differently — for + example, to weight title differently from body — it is necessary + to write a custom trigger. Here is an example using + <span class="application">PL/pgSQL</span> as the trigger language: + +</p><pre class="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 FUNCTION messages_trigger(); +</pre><p> + </p><p> + Keep in mind that it is important to specify the configuration name + explicitly when creating <code class="type">tsvector</code> values inside triggers, + so that the column's contents will not be affected by changes to + <code class="varname">default_text_search_config</code>. Failure to do this is likely to + lead to problems such as search results changing after a dump and restore. + </p></div><div class="sect2" id="TEXTSEARCH-STATISTICS"><div class="titlepage"><div><div><h3 class="title">12.4.4. Gathering Document Statistics</h3></div></div></div><a id="id-1.5.11.7.6.2" class="indexterm"></a><p> + The function <code class="function">ts_stat</code> is useful for checking your + configuration and for finding stop-word candidates. + </p><pre class="synopsis"> +ts_stat(<em class="replaceable"><code>sqlquery</code></em> <code class="type">text</code>, [<span class="optional"> <em class="replaceable"><code>weights</code></em> <code class="type">text</code>, </span>] + OUT <em class="replaceable"><code>word</code></em> <code class="type">text</code>, OUT <em class="replaceable"><code>ndoc</code></em> <code class="type">integer</code>, + OUT <em class="replaceable"><code>nentry</code></em> <code class="type">integer</code>) returns <code class="type">setof record</code> +</pre><p> + <em class="replaceable"><code>sqlquery</code></em> is a text value containing an SQL + query which must return a single <code class="type">tsvector</code> column. + <code class="function">ts_stat</code> executes the query and returns statistics about + each distinct lexeme (word) contained in the <code class="type">tsvector</code> + data. The columns returned are + + </p><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: bullet; "><li class="listitem" style="list-style-type: disc"><p> + <em class="replaceable"><code>word</code></em> <code class="type">text</code> — the value of a lexeme + </p></li><li class="listitem" style="list-style-type: disc"><p> + <em class="replaceable"><code>ndoc</code></em> <code class="type">integer</code> — number of documents + (<code class="type">tsvector</code>s) the word occurred in + </p></li><li class="listitem" style="list-style-type: disc"><p> + <em class="replaceable"><code>nentry</code></em> <code class="type">integer</code> — total number of + occurrences of the word + </p></li></ul></div><p> + + If <em class="replaceable"><code>weights</code></em> is supplied, only occurrences + having one of those weights are counted. + </p><p> + For example, to find the ten most frequent words in a document collection: + +</p><pre class="programlisting"> +SELECT * FROM ts_stat('SELECT vector FROM apod') +ORDER BY nentry DESC, ndoc DESC, word +LIMIT 10; +</pre><p> + + The same, but counting only word occurrences with weight <code class="literal">A</code> + or <code class="literal">B</code>: + +</p><pre class="programlisting"> +SELECT * FROM ts_stat('SELECT vector FROM apod', 'ab') +ORDER BY nentry DESC, ndoc DESC, word +LIMIT 10; +</pre><p> + </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="textsearch-controls.html" title="12.3. Controlling Text Search">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="textsearch.html" title="Chapter 12. Full Text Search">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="textsearch-parsers.html" title="12.5. Parsers">Next</a></td></tr><tr><td width="40%" align="left" valign="top">12.3. Controlling Text Search </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 12.5. Parsers</td></tr></table></div></body></html>
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