pt-duplicate-key-checker – Find duplicate indexes and foreign keys on

MySQL tables.


Usage: pt-duplicate-key-checker [OPTIONS] [DSN]

pt-duplicate-key-checker examines MySQL tables for duplicate or redundant

indexes and foreign keys. Connection options are read from MySQL option


pt-duplicate-key-checker –host host1


Percona Toolkit is mature, proven in the real world, and well tested, but

all database tools can pose a risk to the system and the database server.

Before using this tool, please:

· Read the tool’s documentation

· Review the tool’s known “BUGS”

· Test the tool on a non-production server

· Backup your production server and verify the backups


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.

The output ends with a short summary that includes an estimate of the

total size, in bytes, that the duplicate indexes are using. This is

calculated by multiplying the index length by the number of rows in their

respective tables.


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


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



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


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. If password contains commas they

must be escaped with a backslash: “exam\,ple”

type: string

Create the given PID file. The tool won’t start if the PID file

already exists and the PID it contains is different than the current

PID. However, if the PID file exists and the PID it contains is no

longer running, the tool will overwrite the PID file with the current

PID. The PID file is removed automatically when the tool exits.

short form: -P; type: int

Port number to use for connection.

type: Array

Set the MySQL variables in this comma-separated list of

“variable=value” pairs.

By default, the tool sets:


Variables specified on the command line override these defaults. For

example, specifying “–set-vars wait_timeout=500” overrides the

defaultvalue of 10000.

The tool prints a warning and continues if a variable cannot be set.

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 “–no-sql”.

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.

default: yes

Check for the latest version of Percona Toolkit, MySQL, and other


This is a standard “check for updates automatically” feature, with two

additional features. First, the tool checks the version of other

programs on the local system in addition to its own version. For

example, it checks the version of every MySQL server it connects to,

Perl, and the Perl module DBD::mysql. Second, it checks for and warns

about versions with known problems. For example, MySQL 5.5.25 had a

critical bug and was re-released as 5.5.25a.

Any updates or known problems are printed to STDOUT before the tool’s

normal output. This feature should never interfere with the normal

operation of the tool.

For more information, visit <


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 percona-toolkit manpage for full details.

· A

dsn: charset; copy: yes

Default character set.

· D

dsn: database; copy: yes

Default database.

· F

dsn: mysql_read_default_file; copy: yes

Only read default options from the given file

· h

dsn: host; copy: yes

Connect to host.

· p

dsn: password; copy: yes

Password to use when connecting. If password contains commas they

must be escaped with a backslash: “exam\,ple”

· P

dsn: port; copy: yes

Port number to use for connection.

· S

dsn: mysql_socket; copy: yes

Socket file to use for connection.

· u

dsn: user; copy: yes

User for login if not current user.


The environment variable “PTDEBUG” enables verbose debugging output to

STDERR. To enable debugging and capture all output to a file, run the

tool like:

PTDEBUG=1 pt-duplicate-key-checker … > FILE 2>&1

Be careful: debugging output is voluminous and can generate several

megabytes of output.


You need Perl, DBI, DBD::mysql, and some core packages that ought to be

installed in any reasonably new version of Perl.


For a list of known bugs, see


Please report bugs at <

Include the following information in your bug report:

· Complete command-line used to run the tool

· Tool “–version”

· MySQL version of all servers involved

· Output from the tool including STDERR

· Input files (log/dump/config files, etc.)

If possible, include debugging output by running the tool with “PTDEBUG”;



Visit <
http://www.percona.com/software/percona-toolkit/> to download the

latest release of Percona Toolkit. Or, get the latest release from the

command line:

wget percona.com/get/percona-toolkit.tar.gz

wget percona.com/get/percona-toolkit.rpm

wget percona.com/get/percona-toolkit.deb

You can also get individual tools from the latest release:

wget percona.com/get/TOOL

Replace “TOOL” with the name of any tool.


Baron Schwartz and Daniel Nichter


This tool is part of Percona Toolkit, a collection of advanced command-

line tools for MySQL developed by Percona. Percona Toolkit was forked

from two projects in June, 2011: Maatkit and Aspersa. Those projects were

created by Baron Schwartz and primarily developed by him and Daniel

Nichter. Visit <
http://www.percona.com/software/> to learn about other

free, open-source software from Percona.


This program is copyright 2011-2015 Percona LLC and/or its affiliates,

2007-2011 Baron Schwartz.




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.


pt-duplicate-key-checker 2.2.16

perl v5.20.2 2015-11-06 PT-DUPLICATE-KEY-CHECKER(1p)