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<chapter id="textsearch">

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

<row>

<entry><literal>hword</></entry>

<entry>Hyphenated word, all letters</entry>

<entry><literal>pol&iacute;tico-militar</literal></entry>

<entry><literal>l&oacute;gico-matem&aacute;tica</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>pol&iacute;tico-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>f&iacute;sico</literal> or <literal>qu&iacute;mico</literal>

in the context <literal>f&iacute;sico-qu&iacute;mico</literal></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>

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.