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 2.9, "Installing MySQL from Source".
Note that full-text search is carefully tuned for 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
InnoDB search indexes, and
MyISAM ones. After
changing any of these options, rebuild your
FULLTEXT indexes for the change
to take effect. For example, to make two-character words searchable, you could put the following lines
in an option file:
Then restart the server and rebuild your
FULLTEXT indexes. For
MyISAM tables, note particularly the remarks regarding myisamchk in the instructions following this list.
the default stopword list, set the
ft_stopword_file system 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
The stopword list is free-form, separating stopwords with any nonalphanumeric character such as
newline, space, or comma. 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.
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 MODE
instead, which does not observe the 50% threshold.
To change the operators used for boolean full-text searches on
MyISAM tables, set the
ft_boolean_syntax system variable. (
does not have an equivalent setting.) This variable can be changed while the server is running, but you
must have the
privilege 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.
You can change the set of characters that are considered word characters in several
ways, as described in the following list. After making the modification, rebuild the indexes for each
table that contains any
FULLTEXT indexes. Suppose that you want to treat
the hyphen character ('-') as a word character. Use one of these methods:
Modify the MySQL source: In
'-' 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
array in one of the character set XML files to specify that
is a "letter." Then use the given
character set for your
FULLTEXT indexes. For information about
<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
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:
ALTER TABLE with the
DROP INDEX and
ADD INDEX options to drop and re-create each
index. In some cases, this may be faster than a repair operation.
Each table that contains any
FULLTEXT index 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 repair.
Note that 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 failing.
The problem occurs because these parameters are known only by the server. They are not stored in
MyISAM 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 the
[myisamchk] sections of an option file:
An alternative to using myisamchk for index modification is to use the
OPTIMIZE TABLE, or
TABLE statements. These statements are performed by the server, which knows the proper full-text
parameter values to use.