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


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

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

pt-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.

pt-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:

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.

--ask-pass 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:

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.

--help 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-order 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.

--version Show version and exit.


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.

* 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; see “ENVIRONMENT”.


Visit to download the latest release of Percona Toolkit. Or, get the latest release from the command line:




You can also get individual tools from the latest release:


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 developed by Percona for MySQL support and consulting. Percona Toolkit was forked from two projects in June, 2011: Maatkit and Aspersa. Those projects were created by Baron Schwartz and developed primarily by him and Daniel Nichter, both of whom are employed by Percona. Visit for more software developed by Percona.


pt-duplicate-key-checker 1.0.2

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