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This allows these modules to be installed into a database without superuser privileges (assuming that the DBA or sysadmin has installed the module's files in the expected place). You only need CREATE privilege on the current database, which by default would be available to the database owner. The following modules are marked trusted: btree_gin btree_gist citext cube dict_int earthdistance fuzzystrmatch hstore hstore_plperl intarray isn jsonb_plperl lo ltree pg_trgm pgcrypto seg tablefunc tcn tsm_system_rows tsm_system_time unaccent uuid-ossp In the future we might mark some more modules trusted, but there seems to be no debate about these, and on the whole it seems wise to be conservative with use of this feature to start out with. Discussion: https://postgr.es/m/32315.1580326876@sss.pgh.pa.us
245 lines
6.3 KiB
Plaintext
245 lines
6.3 KiB
Plaintext
<!-- doc/src/sgml/fuzzystrmatch.sgml -->
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<sect1 id="fuzzystrmatch" xreflabel="fuzzystrmatch">
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<title>fuzzystrmatch</title>
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<indexterm zone="fuzzystrmatch">
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<primary>fuzzystrmatch</primary>
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</indexterm>
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<para>
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The <filename>fuzzystrmatch</filename> module provides several
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functions to determine similarities and distance between strings.
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</para>
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<caution>
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<para>
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At present, the <function>soundex</function>, <function>metaphone</function>,
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<function>dmetaphone</function>, and <function>dmetaphone_alt</function> functions do
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not work well with multibyte encodings (such as UTF-8).
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</para>
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</caution>
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<para>
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This module is considered <quote>trusted</quote>, that is, it can be
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installed by non-superusers who have <literal>CREATE</literal> privilege
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on the current database.
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</para>
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<sect2>
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<title>Soundex</title>
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<para>
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The Soundex system is a method of matching similar-sounding names
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by converting them to the same code. It was initially used by the
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United States Census in 1880, 1900, and 1910. Note that Soundex
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is not very useful for non-English names.
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</para>
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<para>
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The <filename>fuzzystrmatch</filename> module provides two functions
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for working with Soundex codes:
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</para>
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<indexterm>
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<primary>soundex</primary>
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</indexterm>
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<indexterm>
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<primary>difference</primary>
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</indexterm>
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<synopsis>
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soundex(text) returns text
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difference(text, text) returns int
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</synopsis>
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<para>
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The <function>soundex</function> function converts a string to its Soundex code.
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The <function>difference</function> function converts two strings to their Soundex
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codes and then reports the number of matching code positions. Since
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Soundex codes have four characters, the result ranges from zero to four,
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with zero being no match and four being an exact match. (Thus, the
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function is misnamed — <function>similarity</function> would have been
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a better name.)
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</para>
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<para>
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Here are some usage examples:
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</para>
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<programlisting>
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SELECT soundex('hello world!');
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SELECT soundex('Anne'), soundex('Ann'), difference('Anne', 'Ann');
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SELECT soundex('Anne'), soundex('Andrew'), difference('Anne', 'Andrew');
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SELECT soundex('Anne'), soundex('Margaret'), difference('Anne', 'Margaret');
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CREATE TABLE s (nm text);
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INSERT INTO s VALUES ('john');
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INSERT INTO s VALUES ('joan');
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INSERT INTO s VALUES ('wobbly');
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INSERT INTO s VALUES ('jack');
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SELECT * FROM s WHERE soundex(nm) = soundex('john');
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SELECT * FROM s WHERE difference(s.nm, 'john') > 2;
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</programlisting>
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</sect2>
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<sect2>
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<title>Levenshtein</title>
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<para>
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This function calculates the Levenshtein distance between two strings:
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</para>
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<indexterm>
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<primary>levenshtein</primary>
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</indexterm>
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<indexterm>
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<primary>levenshtein_less_equal</primary>
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</indexterm>
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<synopsis>
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levenshtein(text source, text target, int ins_cost, int del_cost, int sub_cost) returns int
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levenshtein(text source, text target) returns int
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levenshtein_less_equal(text source, text target, int ins_cost, int del_cost, int sub_cost, int max_d) returns int
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levenshtein_less_equal(text source, text target, int max_d) returns int
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</synopsis>
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<para>
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Both <literal>source</literal> and <literal>target</literal> can be any
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non-null string, with a maximum of 255 characters. The cost parameters
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specify how much to charge for a character insertion, deletion, or
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substitution, respectively. You can omit the cost parameters, as in
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the second version of the function; in that case they all default to 1.
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</para>
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<para>
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<function>levenshtein_less_equal</function> is an accelerated version of the
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Levenshtein function for use when only small distances are of interest.
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If the actual distance is less than or equal to <literal>max_d</literal>,
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then <function>levenshtein_less_equal</function> returns the correct
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distance; otherwise it returns some value greater than <literal>max_d</literal>.
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If <literal>max_d</literal> is negative then the behavior is the same as
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<function>levenshtein</function>.
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</para>
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<para>
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Examples:
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</para>
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<screen>
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test=# SELECT levenshtein('GUMBO', 'GAMBOL');
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levenshtein
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-------------
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2
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(1 row)
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test=# SELECT levenshtein('GUMBO', 'GAMBOL', 2, 1, 1);
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levenshtein
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-------------
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3
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(1 row)
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test=# SELECT levenshtein_less_equal('extensive', 'exhaustive', 2);
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levenshtein_less_equal
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------------------------
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3
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(1 row)
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test=# SELECT levenshtein_less_equal('extensive', 'exhaustive', 4);
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levenshtein_less_equal
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------------------------
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4
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(1 row)
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</screen>
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</sect2>
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<sect2>
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<title>Metaphone</title>
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<para>
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Metaphone, like Soundex, is based on the idea of constructing a
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representative code for an input string. Two strings are then
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deemed similar if they have the same codes.
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</para>
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<para>
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This function calculates the metaphone code of an input string:
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</para>
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<indexterm>
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<primary>metaphone</primary>
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</indexterm>
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<synopsis>
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metaphone(text source, int max_output_length) returns text
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</synopsis>
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<para>
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<literal>source</literal> has to be a non-null string with a maximum of
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255 characters. <literal>max_output_length</literal> sets the maximum
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length of the output metaphone code; if longer, the output is truncated
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to this length.
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</para>
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<para>
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Example:
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</para>
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<screen>
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test=# SELECT metaphone('GUMBO', 4);
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metaphone
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-----------
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KM
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(1 row)
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</screen>
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</sect2>
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<sect2>
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<title>Double Metaphone</title>
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<para>
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The Double Metaphone system computes two <quote>sounds like</quote> strings
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for a given input string — a <quote>primary</quote> and an
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<quote>alternate</quote>. In most cases they are the same, but for non-English
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names especially they can be a bit different, depending on pronunciation.
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These functions compute the primary and alternate codes:
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</para>
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<indexterm>
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<primary>dmetaphone</primary>
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</indexterm>
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<indexterm>
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<primary>dmetaphone_alt</primary>
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</indexterm>
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<synopsis>
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dmetaphone(text source) returns text
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dmetaphone_alt(text source) returns text
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</synopsis>
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<para>
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There is no length limit on the input strings.
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</para>
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<para>
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Example:
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</para>
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<screen>
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test=# SELECT dmetaphone('gumbo');
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dmetaphone
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------------
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KMP
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(1 row)
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</screen>
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</sect2>
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</sect1>
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