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>Appendix F. Additional Supplied Modules</TD
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><H1
CLASS="SECT1"
><A
NAME="FUZZYSTRMATCH"
>F.15. fuzzystrmatch</A
></H1
><P
> The <TT
CLASS="FILENAME"
>fuzzystrmatch</TT
> module provides several
functions to determine similarities and distance between strings.
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
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><TD
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><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> At present, the <CODE
CLASS="FUNCTION"
>soundex</CODE
>, <CODE
CLASS="FUNCTION"
>metaphone</CODE
>,
<CODE
CLASS="FUNCTION"
>dmetaphone</CODE
>, and <CODE
CLASS="FUNCTION"
>dmetaphone_alt</CODE
> functions do
not work well with multibyte encodings (such as UTF-8).
</P
></TD
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></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN164860"
>F.15.1. Soundex</A
></H2
><P
> The Soundex system is a method of matching similar-sounding names
by converting them to the same code. It was initially used by the
United States Census in 1880, 1900, and 1910. Note that Soundex
is not very useful for non-English names.
</P
><P
> The <TT
CLASS="FILENAME"
>fuzzystrmatch</TT
> module provides two functions
for working with Soundex codes:
</P
><PRE
CLASS="SYNOPSIS"
>soundex(text) returns text
difference(text, text) returns int</PRE
><P
> The <CODE
CLASS="FUNCTION"
>soundex</CODE
> function converts a string to its Soundex code.
The <CODE
CLASS="FUNCTION"
>difference</CODE
> function converts two strings to their Soundex
codes and then reports the number of matching code positions. Since
Soundex codes have four characters, the result ranges from zero to four,
with zero being no match and four being an exact match. (Thus, the
function is misnamed — <CODE
CLASS="FUNCTION"
>similarity</CODE
> would have been
a better name.)
</P
><P
> Here are some usage examples:
</P
><PRE
CLASS="PROGRAMLISTING"
>SELECT soundex('hello world!');
SELECT soundex('Anne'), soundex('Ann'), difference('Anne', 'Ann');
SELECT soundex('Anne'), soundex('Andrew'), difference('Anne', 'Andrew');
SELECT soundex('Anne'), soundex('Margaret'), difference('Anne', 'Margaret');
CREATE TABLE s (nm text);
INSERT INTO s VALUES ('john');
INSERT INTO s VALUES ('joan');
INSERT INTO s VALUES ('wobbly');
INSERT INTO s VALUES ('jack');
SELECT * FROM s WHERE soundex(nm) = soundex('john');
SELECT * FROM s WHERE difference(s.nm, 'john') > 2;</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN164876"
>F.15.2. Levenshtein</A
></H2
><P
> This function calculates the Levenshtein distance between two strings:
</P
><PRE
CLASS="SYNOPSIS"
>levenshtein(text source, text target, int ins_cost, int del_cost, int sub_cost) returns int
levenshtein(text source, text target) returns int
levenshtein_less_equal(text source, text target, int ins_cost, int del_cost, int sub_cost, int max_d) returns int
levenshtein_less_equal(text source, text target, int max_d) returns int</PRE
><P
> Both <TT
CLASS="LITERAL"
>source</TT
> and <TT
CLASS="LITERAL"
>target</TT
> can be any
non-null string, with a maximum of 255 bytes. The cost parameters
specify how much to charge for a character insertion, deletion, or
substitution, respectively. You can omit the cost parameters, as in
the second version of the function; in that case they all default to 1.
<TT
CLASS="LITERAL"
>levenshtein_less_equal</TT
> is accelerated version of
levenshtein function for low values of distance. If actual distance
is less or equal then max_d, then <TT
CLASS="LITERAL"
>levenshtein_less_equal</TT
>
returns accurate value of it. Otherwise this function returns value
which is greater than max_d.
</P
><P
> Examples:
</P
><PRE
CLASS="SCREEN"
>test=# SELECT levenshtein('GUMBO', 'GAMBOL');
levenshtein
-------------
2
(1 row)
test=# SELECT levenshtein('GUMBO', 'GAMBOL', 2,1,1);
levenshtein
-------------
3
(1 row)
test=# SELECT levenshtein_less_equal('extensive', 'exhaustive',2);
levenshtein_less_equal
------------------------
3
(1 row)
test=# SELECT levenshtein_less_equal('extensive', 'exhaustive',4);
levenshtein_less_equal
------------------------
4
(1 row)</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN164891"
>F.15.3. Metaphone</A
></H2
><P
> Metaphone, like Soundex, is based on the idea of constructing a
representative code for an input string. Two strings are then
deemed similar if they have the same codes.
</P
><P
> This function calculates the metaphone code of an input string:
</P
><PRE
CLASS="SYNOPSIS"
>metaphone(text source, int max_output_length) returns text</PRE
><P
> <TT
CLASS="LITERAL"
>source</TT
> has to be a non-null string with a maximum of
255 characters. <TT
CLASS="LITERAL"
>max_output_length</TT
> sets the maximum
length of the output metaphone code; if longer, the output is truncated
to this length.
</P
><P
> Example:
</P
><PRE
CLASS="SCREEN"
>test=# SELECT metaphone('GUMBO', 4);
metaphone
-----------
KM
(1 row)</PRE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="AEN164903"
>F.15.4. Double Metaphone</A
></H2
><P
> The Double Metaphone system computes two <SPAN
CLASS="QUOTE"
>"sounds like"</SPAN
> strings
for a given input string — a <SPAN
CLASS="QUOTE"
>"primary"</SPAN
> and an
<SPAN
CLASS="QUOTE"
>"alternate"</SPAN
>. In most cases they are the same, but for non-English
names especially they can be a bit different, depending on pronunciation.
These functions compute the primary and alternate codes:
</P
><PRE
CLASS="SYNOPSIS"
>dmetaphone(text source) returns text
dmetaphone_alt(text source) returns text</PRE
><P
> There is no length limit on the input strings.
</P
><P
> Example:
</P
><PRE
CLASS="SCREEN"
>test=# select dmetaphone('gumbo');
dmetaphone
------------
KMP
(1 row)</PRE
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