mk-index-usage.1p man page

mk-index-usage — Read queries from a log and analyze how they use indexes.


Usage: mk-index-usage [OPTION...] [FILE...]

mk-index-usage reads queries from logs and analyzes how they use indexes.

Analyze queries in slow.log and print reports:

  mk-index-usage /path/to/slow.log --host localhost

Disable reports and save results to mk database for later analysis:

  mk-index-usage slow.log --no-report --save-results-database mk


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.

This tool is read-only unless you use "--save-results-database".  It reads a log of queries and EXPLAIN them.  It also gathers information about all tables in all databases.  It should be very low-risk.

At the time of this release, we know of no bugs that could cause serious harm to users.

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 tool connects to a MySQL database server, reads through a query log, and uses EXPLAIN to ask MySQL how it will use each query.  When it is finished, it prints out a report on indexes that the queries didn't use.

The query log needs to be in MySQL's slow query log format.  If you need to input a different format, you can use mk-query-digest to translate the formats.  If you don't specify a filename, the tool reads from STDIN.

The tool runs two stages.  In the first stage, the tool takes inventory of all the tables and indexes in your database, so it can compare the existing indexes to those that were actually used by the queries in the log.  In the second stage, it runs EXPLAIN on each query in the query log.  It uses separate database connections to inventory the tables and run EXPLAIN, so it opens two connections to the database.

If a query is not a SELECT, it tries to transform it to a roughly equivalent SELECT query so it can be EXPLAINed.  This is not a perfect process, but it is good enough to be useful.

The tool skips the EXPLAIN step for queries that are exact duplicates of those seen before.  It assumes that the same query will generate the same EXPLAIN plan as it did previously (usually a safe assumption, and generally good for performance), and simply increments the count of times that the indexes were used.  However, queries that have the same fingerprint but different checksums will be re-EXPLAINed.  Queries that have different literal constants can have different execution plans, and this is important to measure.

After EXPLAIN-ing the query, it is necessary to try to map aliases in the query back to the original table names.  For example, consider the EXPLAIN plan for the following query:

  SELECT * FROM tbl1 AS foo;

The EXPLAIN output will show access to table "foo", and that must be translated back to "tbl1".  This process involves complex parsing.  It is generally very accurate, but there is some chance that it might not work right.  If you find cases where it fails, submit a bug report and a reproducible test case.

Queries that cannot be EXPLAINed will cause all subsequent queries with the same fingerprint to be blacklisted.  This is to reduce the work they cause, and prevent them from continuing to print error messages.  However, at least in this stage of the tool's development, it is my opinion that it's not a good idea to preemptively silence these, or prevent them from being EXPLAINed at all.  I am looking for lots of feedback on how to improve things like the query parsing.  So please submit your test cases based on the errors the tool prints!


After it reads all the events in the log, the tool prints out DROP statements for every index that was not used.  It skips indexes for tables that were never accessed by any queries in the log, to avoid false-positive results.

If you don't specify "--quiet", the tool also outputs warnings about statements that cannot be EXPLAINed and similar.  These go to standard error.

Progress reports are enabled by default (see "--progress").  These also go to standard error.


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


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.


type: Array

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


Create the "--save-results-database" if it does not exist.

If the "--save-results-database" already exists and this option is specified, the database is used and the necessary tables are created if they do not already exist.


Create views for "--save-results-database" example queries.

Several example queries are given for querying the tables in the "--save-results-database".  These example queries are, by default, created as views.  Specifying "--no-create-views" prevents these views from being created.


short form: -D; type: string

The database to use for the connection.


short form: -d; type: hash

Only get tables and indexes from this comma-separated list of databases.


type: string

Only get tables and indexes from database whose names match this Perl regex.


short form: -F; type: string

Only read mysql options from the given file.  You must give an absolute pathname.


type: Hash; default: non-unique

Suggest dropping only these types of unused indexes.

By default mk-index-usage will only suggest to drop unused secondary indexes, not primary or unique indexes.  You can specify which types of unused indexes the tool suggests to drop: primary, unique, non-unique, all.

A separate "ALTER TABLE" statement for each type is printed.  So if you specify "--drop all" and there is a primary key and a non-unique index, the "ALTER TABLE ... DROP" for each will be printed on separate lines.


Drop and re-create all pre-existing tables in the "--save-results-database". This allows information from previous runs to be removed before the current run.


Show help and exit.


short form: -h; type: string

Connect to host.


type: Hash

Ignore this comma-separated list of databases.


type: string

Ignore databases whose names match this Perl regex.


type: Hash

Ignore this comma-separated list of table names.

Table names may be qualified with the database name.


type: string

Ignore tables whose names match the Perl regex.


short form: -p; type: string

Password to use when connecting.


short form: -P; type: int

Port number to use for connection.


type: array; default: time,30

Print progress reports to STDERR.  The value is a comma-separated list with two parts.  The first part can be percentage, time, or iterations; the second part specifies how often an update should be printed, in percentage, seconds, or number of iterations.


short form: -q

Do not print any warnings.  Also disables "--progress".


default: yes

Print the reports for "--report-format".

You may want to disable the reports by specifying "--no-report" if, for example, you also specify "--save-results-database" and you only want to query the results tables later.


type: Array; default: drop_unused_indexes

Right now there is only one report: drop_unused_indexes.  This report prints SQL statements for dropping any unused indexes.  See also "--drop".

See also "--[no]report".


type: DSN

Save results to tables in this database.  Information about indexes, queries, tables and their usage is stored in several tables in the specified database. The tables are auto-created if they do not exist.  If the database doesn't exist, it can be auto-created with "--create-save-results-database".  In this case the connection is initially created with no default database, then after the database is created, it is USE'ed.

mk-index-usage executes INSERT statements to save the results.  Therefore, you should be careful if you use this feature on a production server.  It might increase load, or cause trouble if you don't want the server to be written to, or so on.

This is a new feature.  It may change in future releases.

After a run, you can query the usage tables to answer various questions about index usage.  The tables have the following CREATE TABLE definitions:


    db           VARCHAR(64) NOT NULL,
    tbl          VARCHAR(64) NOT NULL,
    idx          VARCHAR(64) NOT NULL,
    PRIMARY KEY  (db, tbl, idx)


    query_id     BIGINT UNSIGNED NOT NULL,
    fingerprint  TEXT NOT NULL,
    sample       TEXT NOT NULL,
    PRIMARY KEY  (query_id)


    db           VARCHAR(64) NOT NULL,
    tbl          VARCHAR(64) NOT NULL,
    PRIMARY KEY  (db, tbl)


    query_id      BIGINT UNSIGNED NOT NULL,
    db            VARCHAR(64) NOT NULL,
    tbl           VARCHAR(64) NOT NULL,
    idx           VARCHAR(64) NOT NULL,
    UNIQUE INDEX  (query_id, db, tbl, idx)


  CREATE TABLE IF NOT EXISTS index_alternatives (
    query_id      BIGINT UNSIGNED NOT NULL, -- This query used
    db            VARCHAR(64) NOT NULL,     -- this index, but...
    tbl           VARCHAR(64) NOT NULL,     --
    idx           VARCHAR(64) NOT NULL,     --
    alt_idx       VARCHAR(64) NOT NULL,     -- was an alternative
    UNIQUE INDEX  (query_id, db, tbl, idx, alt_idx),
    INDEX         (db, tbl, idx),
    INDEX         (db, tbl, alt_idx)

The following are some queries you can run against these tables to answer common questions you might have.  Each query is also created as a view (with MySQL v5.0 and newer) if "--[no]create-views" is true (it is by default). The view names are the strings after the "MAGIC_view_" prefix.

Question: which queries sometimes use different indexes, and what fraction of the time is each index chosen?  MAGIC_view_query_uses_several_indexes:

 SELECT iu.query_id, CONCAT_WS('.', iu.db, iu.tbl, iu.idx) AS idx,
    variations, iu.cnt, iu.cnt / total_cnt * 100 AS pct
 FROM index_usage AS iu
       SELECT query_id, db, tbl, SUM(cnt) AS total_cnt,
         COUNT(*) AS variations
       FROM index_usage
       GROUP BY query_id, db, tbl
       HAVING COUNT(*) > 1
    ) AS qv USING(query_id, db, tbl);

Question: which indexes have lots of alternatives, i.e. are chosen instead of other indexes, and for what queries?  MAGIC_view_index_has_alternates:

 SELECT CONCAT_WS('.', db, tbl, idx) AS idx_chosen,
    GROUP_CONCAT(DISTINCT alt_idx) AS alternatives,
    GROUP_CONCAT(DISTINCT query_id) AS queries, SUM(cnt) AS cnt
 FROM index_alternatives
 GROUP BY db, tbl, idx

Question: which indexes are considered as alternates for other indexes, and for what queries?  MAGIC_view_index_alternates:

 SELECT CONCAT_WS('.', db, tbl, alt_idx) AS idx_considered,
    GROUP_CONCAT(DISTINCT idx) AS alternative_to,
    GROUP_CONCAT(DISTINCT query_id) AS queries, SUM(cnt) AS cnt
 FROM index_alternatives
 GROUP BY db, tbl, alt_idx

Question: which of those are never chosen by any queries, and are therefore superfluous?  MAGIC_view_unused_index_alternates:

 SELECT CONCAT_WS('.', i.db, i.tbl, i.idx) AS idx,
    alt.alternative_to, alt.queries, alt.cnt
 FROM indexes AS i
       SELECT db, tbl, alt_idx, GROUP_CONCAT(DISTINCT idx) AS alternative_to,
          GROUP_CONCAT(DISTINCT query_id) AS queries, SUM(cnt) AS cnt
       FROM index_alternatives
       GROUP BY db, tbl, alt_idx
       HAVING COUNT(*) > 1
    ) AS alt ON i.db = alt.db AND i.tbl = alt.tbl
      AND i.idx = alt.alt_idx
 WHERE i.cnt = 0;

Question: given a table, which indexes were used, by how many queries, with how many distinct fingerprints?  Were there alternatives?  Which indexes were not used?  You can edit the following query's SELECT list to also see the query IDs in question.  MAGIC_view_index_usage:

 SELECT i.idx, iu.usage_cnt, iu.usage_total,
    ia.alt_cnt, ia.alt_total
 FROM indexes AS i
       SELECT db, tbl, idx, COUNT(*) AS usage_cnt,
          SUM(cnt) AS usage_total, GROUP_CONCAT(query_id) AS used_by
       FROM index_usage
       GROUP BY db, tbl, idx
    ) AS iu ON i.db=iu.db AND i.tbl=iu.tbl AND i.idx = iu.idx
       SELECT db, tbl, idx, COUNT(*) AS alt_cnt,
          SUM(cnt) AS alt_total,
          GROUP_CONCAT(query_id) AS alt_queries
       FROM index_alternatives
       GROUP BY db, tbl, idx
    ) AS ia ON i.db=ia.db AND i.tbl=ia.tbl AND i.idx = ia.idx;

Question: which indexes on a given table are vital for at least one query (there is no alternative)?  MAGIC_view_required_indexes:

   SELECT i.db, i.tbl, i.idx, no_alt.queries
   FROM indexes AS i
      INNER JOIN (
         SELECT iu.db, iu.tbl, iu.idx,
            GROUP_CONCAT(iu.query_id) AS queries
         FROM index_usage AS iu
            LEFT OUTER JOIN index_alternatives AS ia
               USING(db, tbl, idx)
         WHERE ia.db IS NULL
         GROUP BY iu.db, iu.tbl, iu.idx
      ) AS no_alt ON no_alt.db = i.db AND no_alt.tbl = i.tbl
         AND no_alt.idx = i.idx
   ORDER BY i.db, i.tbl, i.idx, no_alt.queries;

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.


short form: -t; type: hash

Only get indexes from this comma-separated list of tables.


type: string

Only get indexes from tables whose names match this Perl regex.


short form: -u; type: string

User for login if not current user.


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 <>, 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.


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

   MKDEBUG=1 mk-....

System Requirements

You need Perl and some core packages that ought to be installed in any reasonably new version of Perl.


For a list of known bugs see <>.

Please use Google Code Issues and Groups to report bugs or request support: <>.  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 2010-2011 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 0.9.4 Distrib 7540 $Revision: 7477 $.


2011-06-08 perl v5.24.1 User Contributed Perl Documentation