Langue: en

Version: 2008-12-29 (fedora - 04/07/09)

Section: 1 (Commandes utilisateur)


mk-duplicate-key-checker - Find possible duplicate indexes and foreign keys on MySQL tables.


This program examines the output of SHOW CREATE TABLE on MySQL tables, and if it finds indexes that cover the same columns as another index in the same order, or cover an exact leftmost prefix of another index, it prints out the suspicious indexes. By default, indexes must be of the same type, so a BTREE index is not a duplicate of a FULLTEXT index, even if they have the same colums. You can override this.

It also looks for duplicate foreign keys. A duplicate foreign key covers the same columns as another in the same table, and references the same parent table.


You can download Maatkit from Google Code at <>, or you can get any of the tools easily with a command like the following:

Where "toolname" can be replaced with the name (or fragment of a name) of any of the Maatkit tools. Once downloaded, they're ready to run; no installation is needed. The first URL gets the latest released version of the tool, and the second gets the latest trunk code from Subversion.


short form: -s

Print only once, instead of one DB at a time.

Prints everything it finds in one chunk. The default is to print a database at a time.

short form: -a

Compare indexes with different structs (BTREE, HASH, etc).

By default this is disabled, because a BTREE index that covers the same columns as a FULLTEXT index is not really a duplicate, for example.

Prompt for a password when connecting to MySQL.
short form: -A; type: string

Default character set.

Enables character set settings in Perl and MySQL. If the value is "utf8", sets Perl's binmode on STDOUT to utf8, passes the "mysql_enable_utf8" option to DBD::mysql, and runs "SET NAMES UTF8" after connecting to MySQL. Any other value sets binmode on STDOUT without the utf8 layer, and runs "SET NAMES" after connecting to MySQL.

short form: -c

PK columns appended to secondary key is duplicate.

Detects when a suffix of a secondary key is a leftmost prefix of the primary key, and treats it as a duplicate key. Only detects this condition on storage engines whose primary keys are clustered (currently InnoDB and solidDB).

Clustered storage engines append the primary key columns to the leaf nodes of all secondary keys anyway, so you might consider it redundant to have them appear in the internal nodes as well. Of course, you may also want them in the internal nodes, because just having them at the leaf nodes won't help for some queries. It does help for covering index queries, however.

Here's an example of a key that is considered redundant with this option:

   PRIMARY KEY  (`a`)
   KEY `b` (`b`,`a`)
short form: -d; type: hash

Check only this comma-separated list of databases.

short form: -F; type: string

Only read mysql options from the given file.

You must give an absolute pathname.

short form: -e; type: hash

Do only tables whose storage engine is in this comma-separated list.

short form: -f; type: string; default: fk

Do f=foreign keys, k=keys or fk=check both.

short form: -h; type: string

Connect to host.

short form: -g; type: Hash

Ignore this comma-separated list of databases.

short form: -E; type: Hash

Ignore this comma-separated list of storage engines.

Ignore index order so KEY(a,b) duplicates KEY(b,a).
short form: -n; type: Hash

Ignore this comma-separated list of tables.

Table names may be qualified with the database name.

short form: -p; type: string

Password to use when connecting.

short form: -P; type: int

Port number to use for connection.

type: string

Set these MySQL variables (default wait_timeout=10000).

Specify any variables you want to be set immediately after connecting to MySQL. These will be included in a "SET" command.

short form: -S; type: string

Socket file to use for connection.

short form: -b

Print output separated with tabs instead of whitespace-aligned.

See ``OUTPUT'' for details.

short form: -t; type: hash

Check only this comma-separated list of tables.

Table names may be qualified with the database name.

short form: -u; type: string

User for login if not current user.

short form: -v

Output all keys and/or foreign keys found, not just redundant ones.


Output is to STDOUT, one line per server and table, with header lines for each database. I tried to make the output easy to process with awk. For this reason columns are always present. If there's no value, the script prints 'NULL'. Output is sorted by database and table.

The columns in the output are as follows.

The database the table is in.
The table name.
The table's storage engine.
The index or constraint's name, e.g. `tbl_ibfk_3` (the default InnoDB name for the third foreign key on a table named tbl).
'KEY' for indexes, 'FK' for foreign keys.
The type of index: BTREE, FULLTEXT, HASH etc. By default MySQL's indexes are BTREE in most cases. This does not apply to foreign keys.
The parent table to which the foreign key constraint refers. This does not apply to indexes.
The columns included in the index or foreign key constraint. For indexes, this column list is output verbatim, as shown in SHOW CREATE TABLE. For foreign keys, the columns are ordered so string comparison can find duplicates, since column order in a foreign key is immaterial.


The environment variable "MKDEBUG" enables verbose debugging output in all of the Maatkit tools:
    MKDEBUG=1 mk-....


You need the following Perl modules: DBI and DBD::mysql.


Please use Google Code Issues and Groups to report bugs or request support: <>.

Please include the complete command-line used to reproduce the problem you are seeing, the version of all MySQL servers involved, the complete output of the tool when run with ``--version'', and if possible, debugging output produced by running with the "MKDEBUG=1" environment variable.


This program is copyright 2007-2008 Baron Schwartz. Feedback and improvements are welcome.


This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2; OR the Perl Artistic License. On UNIX and similar systems, you can issue `man perlgpl' or `man perlartistic' to read these licenses.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.


Baron Schwartz.


This manual page documents Ver 1.1.9 Distrib 2725 $Revision: 2463 $.