-<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.26 2007/10/25 13:06:35 alvherre Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.27 2007/10/27 00:19:45 tgl Exp $ -->
<chapter id="textsearch">
<title id="textsearch-title">Full Text Search</title>
<row>
<entry><literal>hword</></entry>
<entry>Hyphenated word, all letters</entry>
- <entry><literal>político-militar</literal></entry>
+ <entry><literal>lógico-matemática</literal></entry>
</row>
<row>
<entry><literal>numhword</></entry>
<row>
<entry><literal>hword_asciipart</></entry>
<entry>Hyphenated word part, all ASCII</entry>
- <entry><literal>militar</literal> in the context
- <literal>político-militar</literal>, or <literal>postgresql</literal> in the context <literal>postgresql-beta1</literal></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>físico</literal> or <literal>químico</literal>
- in the context <literal>físico-químico</literal></entry>
+ <entry><literal>lógico</literal> or <literal>matemática</literal>
+ in the context <literal>lógico-matemática</literal></entry>
</row>
<row>
<entry><literal>hword_numpart</></entry>
instructive example:
<programlisting>
-SELECT alias, description, token FROM ts_debug('http://foo.com/stuff/index.html');
- alias | description | token
-----------+---------------+--------------------------
+SELECT alias, description, token FROM ts_debug('http://example.com/stuff/index.html');
+ alias | description | token
+----------+---------------+------------------------------
protocol | Protocol head | http://
- url | URL | foo.com/stuff/index.html
- host | Host | foo.com
+ url | URL | example.com/stuff/index.html
+ host | Host | example.com
uri | URI | /stuff/index.html
</programlisting>
</para>
</para>
<para>
- A GiST index is <firstterm>lossy</firstterm>, meaning it is necessary
- to check the actual table row to eliminate false matches.
+ 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:
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, and if all words in
+ 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 since random access to table
- records is slow; this limits the usefulness of GiST indexes. The
+ 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.