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mk-duplicate-key-checker.1p - Man Page

Find duplicate indexes and foreign keys on MySQL tables.


Usage: mk-duplicate-key-checker [OPTION...] [DSN]

mk-duplicate-key-checker examines MySQL tables for duplicate or redundant indexes and foreign keys.  Connection options are read from MySQL option files.

   mk-duplicate-key-checker --host host1


The following section is included to inform users about the potential risks, whether known or unknown, of using this tool.  The two main categories of risks are those created by the nature of the tool (e.g. read-only tools vs. read-write tools) and those created by bugs.

mk-duplicate-key-checker is a read-only tool that executes SHOW CREATE TABLE and related queries to inspect table structures, and thus is very low-risk.

At the time of this release, there is an unconfirmed bug that causes the tool to crash.

The authoritative source for updated information is always the online issue tracking system.  Issues that affect this tool will be marked as such.  You can see a list of such issues at the following URL: <http://www.maatkit.org/bugs/mk-duplicate-key-checker>.

See also "Bugs" for more information on filing bugs and getting help.


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 columns.  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.


This tool accepts additional command-line arguments.  Refer to the "Synopsis" and usage information for details.


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.  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.


default: yes

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`)

The use of such indexes is rather subtle.  For example, suppose you have the following query:


This query will do a filesort if we remove the index on b,a.  But if we shorten the index on b,a to just b and also remove the ORDER BY, the query should return the same results.

The tool suggests shortening duplicate clustered keys by dropping the key and re-adding it without the primary key prefix.  The shortened clustered key may still duplicate another key, but the tool cannot currently detect when this happens without being ran a second time to re-check the newly shortened clustered keys.  Therefore, if you shorten any duplicate clustered keys, you should run the tool again.


type: Array

Read this comma-separated list of config files; if specified, this must be the first option on the command line.


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

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


Show help and exit.


short form: -h; type: string

Connect to host.


type: Hash

Ignore this comma-separated list of databases.


type: Hash

Ignore this comma-separated list of storage engines.


Ignore index order so KEY(a,b) duplicates KEY(b,a).


type: Hash

Ignore this comma-separated list of tables.  Table names may be qualified with the database name.


type: string; default: fk

Check for duplicate f=foreign keys, k=keys or fk=both.


short form: -p; type: string

Password to use when connecting.


type: string

Create the given PID file.  The file contains the process ID of the script. The PID file is removed when the script exits.  Before starting, the script checks if the PID file already exists.  If it does not, then the script creates and writes its own PID to it.  If it does, then the script checks the following: if the file contains a PID and a process is running with that PID, then the script dies; or, if there is no process running with that PID, then the script overwrites the file with its own PID and starts; else, if the file contains no PID, then the script dies.


short form: -P; type: int

Port number to use for connection.


type: string; default: wait_timeout=10000

Set these MySQL variables.  Immediately after connecting to MySQL, this string will be appended to SET and executed.


short form: -S; type: string

Socket file to use for connection.


default: yes

Print DROP KEY statement for each duplicate key.  By default an ALTER TABLE DROP KEY statement is printed below each duplicate key so that, if you want to remove the duplicate key, you can copy-paste the statement into MySQL.

To disable printing these statements, specify --nosql.


default: yes

Print summary of indexes at end of output.


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.


Show version and exit.

DSN Options

These DSN options are used to create a DSN.  Each option is given like option=value.  The options are case-sensitive, so P and p are not the same option.  There cannot be whitespace before or after the = and if the value contains whitespace it must be quoted.  DSN options are comma-separated.  See the maatkit manpage for full details.


You can download Maatkit from Google Code at <http://code.google.com/p/maatkit/>, or you can get any of the tools easily with a command like the following:

   wget http://www.maatkit.org/get/toolname
   wget http://www.maatkit.org/trunk/toolname

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.


The environment variable MKDEBUG enables verbose debugging output in all of the Maatkit tools:

   MKDEBUG=1 mk-....

System Requirements

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


For a list of known bugs see <http://www.maatkit.org/bugs/mk-duplicate-key-checker>.

Please use Google Code Issues and Groups to report bugs or request support: <http://code.google.com/p/maatkit/>.  You can also join #maatkit on Freenode to discuss Maatkit.

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.

Copyright, License and Warranty

This program is copyright 2007-2011 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, Daniel Nichter

About Maatkit

This tool is part of Maatkit, a toolkit for power users of MySQL.  Maatkit was created by Baron Schwartz; Baron and Daniel Nichter are the primary code contributors.  Both are employed by Percona.  Financial support for Maatkit development is primarily provided by Percona and its clients.


This manual page documents Ver 1.2.15 Distrib 7540 $Revision: 7477 $.


2024-01-25 perl v5.38.2 User Contributed Perl Documentation