MySQL's full-text search capability has few user-tunable parameters. You can exert more control over full-text searching behavior if you have a MySQL source distribution because some changes require source code modifications. See Section 2.17, “Installing MySQL from Source”.
Full-text search is carefully tuned for the most effectiveness. Modifying the default behavior in most cases can actually decrease effectiveness. Do not alter the MySQL sources unless you know what you are doing.
Most full-text variables described in this section must be set at server startup time. A server restart is required to change them; they cannot be modified while the server is running.
Some variable changes require that you rebuild the
FULLTEXT indexes in your tables. Instructions
for doing so are given later in this section.
The minimum and maximum lengths of words to be indexed are defined by the
ft_max_word_lensystem variables. (See Section 5.1.4, “Server System Variables”.) The default minimum value is four characters; the default maximum is version dependent. If you change either value, you must rebuild your
FULLTEXTindexes. For example, if you want three-character words to be searchable, you can set the
ft_min_word_lenvariable by putting the following lines in an option file:
Then restart the server and rebuild your
FULLTEXTindexes. Note particularly the remarks regarding myisamchk in the instructions following this list.
To override the default stopword list, set the
ft_stopword_filesystem variable. (See Section 5.1.4, “Server System Variables”.) The variable value should be the path name of the file containing the stopword list, or the empty string to disable stopword filtering. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. After changing the value of this variable or the contents of the stopword file, restart the server and rebuild your
The stopword list is free-form. That is, you may use any nonalphanumeric character such as newline, space, or comma to separate stopwords. Exceptions are the underscore character (“
_”) and a single apostrophe (“
'”) which are treated as part of a word. The character set of the stopword list is the server's default character set; see Section 10.1.3.1, “Server Character Set and Collation”.
The 50% threshold for natural language searches is determined by the particular weighting scheme chosen. To disable it, look for the following line in
#define GWS_IN_USE GWS_PROB
Change that line to this:
#define GWS_IN_USE GWS_FREQ
Then recompile MySQL. There is no need to rebuild the indexes in this case.Note
By making this change, you severely decrease MySQL's ability to provide adequate relevance values for the
MATCH()function. If you really need to search for such common words, it would be better to search using
IN BOOLEAN MODEinstead, which does not observe the 50% threshold.
To change the operators used for boolean full-text searches, set the
ft_boolean_syntaxsystem variable. This variable can be changed while the server is running, but you must have the
SUPERprivilege to do so. No rebuilding of indexes is necessary in this case. See Section 5.1.4, “Server System Variables”, which describes the rules governing how to set this variable.
If you want to change the set of characters that are considered word characters, you can do so in several ways, as described in the following list. After making the modification, you must rebuild the indexes for each table that contains any
FULLTEXTindexes. Suppose that you want to treat the hyphen character ('-') as a word character. Use one of these methods:
Modify the MySQL source: In
myisam/ftdefs.h, see the
'-'to one of those macros and recompile MySQL.
Modify a character set file: This requires no recompilation. The
true_word_char()macro uses a “character type” table to distinguish letters and numbers from other characters. . You can edit the contents of the
<ctype><map>array in one of the character set XML files to specify that
'-'is a “letter.” Then use the given character set for your
FULLTEXTindexes. For information about the
<ctype><map>array format, see Section 10.3.1, “Character Definition Arrays”.
Add a new collation for the character set used by the indexed columns, and alter the columns to use that collation. For general information about adding collations, see Section 10.4, “Adding a Collation to a Character Set”. For an example specific to full-text indexing, see Section 12.9.7, “Adding a Collation for Full-Text Indexing”.
If you modify full-text variables that affect indexing
ft_stopword_file), or if you
change the stopword file itself, you must rebuild your
FULLTEXT indexes after making the changes and
restarting the server. To rebuild the indexes in this case, it
is sufficient to do a
QUICK repair operation:
DROP INDEX and
INDEX options to drop and re-create each
FULLTEXT index. In some cases, this may be
faster than a repair operation.
Each table that contains any
must be repaired as just shown. Otherwise, queries for the table
may yield incorrect results, and modifications to the table will
cause the server to see the table as corrupt and in need of
If you use myisamchk to perform an operation
that modifies table indexes (such as repair or analyze), the
FULLTEXT indexes are rebuilt using the
default full-text parameter values for
minimum word length, maximum word length, and stopword file
unless you specify otherwise. This can result in queries
The problem occurs because these parameters are known only by
the server. They are not stored in
index files. To avoid the problem if you have modified the
minimum or maximum word length or stopword file values used by
the server, specify the same
ft_stopword_file values for
myisamchk that you use for
mysqld. For example, if you have set the
minimum word length to 3, you can repair a table with
myisamchk like this:
myisamchk --recover --ft_min_word_len=3
To ensure that myisamchk and the server use
the same values for full-text parameters, place each one in both
[myisamchk] sections of an option file:
[mysqld] ft_min_word_len=3 [myisamchk] ft_min_word_len=3
An alternative to using myisamchk for index
modification is to use the
OPTIMIZE TABLE, or
ALTER TABLE statements. These
statements are performed by the server, which knows the proper
full-text parameter values to use.