MySQL 9.1 Release Notes
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 2.8, “Installing MySQL from Source”.
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_ft_min_token_size and
innodb_ft_max_token_size for
InnoDB search indexes, and
ft_min_word_len and
ft_max_word_len for
MyISAM ones.
Note
Minimum and maximum word length full-text parameters do not
apply to FULLTEXT indexes created using
the ngram parser. ngram token size is defined by the
ngram_token_size option.
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:
[mysqld]
innodb_ft_min_token_size=2
ft_min_word_len=2
Then restart the server and rebuild your
FULLTEXT indexes. For
MyISAM tables, note the remarks regarding
myisamchk in the instructions that follow
for rebuilding MyISAM full-text indexes.
For MyISAM search indexes, the 50%
threshold for natural language searches is determined by the
particular weighting scheme chosen. To disable it, look for
the following line in
storage/myisam/ftdefs.h:
#define GWS_IN_USE GWS_PROBChange that line to this:
#define GWS_IN_USE GWS_FREQThen 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 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. (InnoDB does not have an
equivalent setting.) This variable can be changed while the
server is running, but you must have privileges sufficient to
set global system variables (see
Section 7.1.9.1, “System Variable Privileges”). No rebuilding
of indexes is necessary in this case.
For the built-in full-text parser, 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
storage/innobase/handler/ha_innodb.cc(forInnoDB), or instorage/myisam/ftdefs.h(forMyISAM), see thetrue_word_char()andmisc_word_char()macros. Add'-'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 yourFULLTEXTindexes. For information about the<ctype><map>array format, see Section 12.13.1, “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 12.14, “Adding a Collation to a Character Set”. For an example specific to full-text indexing, see Section 14.9.7, “Adding a User-Defined Collation for Full-Text Indexing”.
For the changes to take effect, FULLTEXT
indexes must be rebuilt after modifying any of the following
full-text index variables:
innodb_ft_min_token_size;
innodb_ft_max_token_size;
innodb_ft_server_stopword_table;
innodb_ft_user_stopword_table;
innodb_ft_enable_stopword;
ngram_token_size. Modifying
innodb_ft_min_token_size,
innodb_ft_max_token_size, or
ngram_token_size requires
restarting the server.
To rebuild FULLTEXT indexes for an
InnoDB table, use
ALTER TABLE with the
DROP INDEX and ADD INDEX
options to drop and re-create each index.
Running OPTIMIZE TABLE on a
table with a full-text index rebuilds the full-text index,
removing deleted Document IDs and consolidating multiple
entries for the same word, where possible.
To optimize a full-text index, enable
innodb_optimize_fulltext_only
and run OPTIMIZE TABLE.
mysql> set GLOBAL innodb_optimize_fulltext_only=ON;
Query OK, 0 rows affected (0.01 sec)
mysql> OPTIMIZE TABLE opening_lines;
+--------------------+----------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+--------------------+----------+----------+----------+
| test.opening_lines | optimize | status | OK |
+--------------------+----------+----------+----------+
1 row in set (0.01 sec)
To avoid lengthy rebuild times for full-text indexes on large
tables, you can use the
innodb_ft_num_word_optimize
option to perform the optimization in stages. The
innodb_ft_num_word_optimize option defines
the number of words that are optimized each time
OPTIMIZE TABLE is run. The
default setting is 2000, which means that 2000 words are
optimized each time OPTIMIZE
TABLE is run. Subsequent
OPTIMIZE TABLE operations
continue from where the preceding
OPTIMIZE TABLE operation ended.
If you modify full-text variables that affect indexing
(ft_min_word_len,
ft_max_word_len, or
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 FULLTEXT indexes for a
MyISAM table, it is sufficient to do a
QUICK repair operation:
mysql> REPAIR TABLE tbl_name QUICK;
Alternatively, use ALTER TABLE
as just described. 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 causes the server to see the table as corrupt and in
need of repair.
If you use myisamchk to perform an
operation that modifies MyISAM 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_min_word_len,
ft_max_word_len, and
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 tbl_name.MYI
To ensure that myisamchk and the server use
the same values for full-text parameters, place each one in
both the [mysqld] and
[myisamchk] sections of an option file:
[mysqld]
ft_min_word_len=3
[myisamchk]
ft_min_word_len=3
An alternative to using myisamchk for
MyISAM table index modification is to use
the REPAIR TABLE,
ANALYZE TABLE,
OPTIMIZE TABLE, or
ALTER TABLE statements. These
statements are performed by the server, which knows the proper
full-text parameter values to use.