pt-query-advisor - Analyze queries and advise on possible problems.


Usage: pt-query-advisor [OPTION...] [FILE]

pt-query-advisor analyzes queries and advises on possible problems. Queries are given either by specifying slowlog files, –query, or –review.

Analyze all queries in a slow log:

pt-query-advisor /path/to/slow-query.log

Analyze all queires in a general log:

pt-query-advisor --type genlog mysql.log

Get queries from tcpdump using pt-query-digest:

pt-query-digest --type tcpdump.txt --print --no-report | pt-query-advisor


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-query-advisor simply reads queries and examines them, and is thus very low risk.

At the time of this release there is a bug that may cause an infinite (or very long) loop when parsing very large queries.

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.


pt-query-advisor examines queries and applies rules to them, trying to find queries that look bad according to the rules. It reports on queries that match the rules, so you can find bad practices or hidden problems in your SQL. By default, it accepts a MySQL slow query log as input.


These are the rules that pt-query-advisor will apply to the queries it examines. Each rule has three bits of information: an ID, a severity and a description.

The rule’s ID is its identifier. We use a seven-character ID, and the naming convention is three characters, a period, and a three-digit number. The first three characters are sort of an abbreviation of the general class of the rule. For example, ALI.001 is some rule related to how the query uses aliases.

The rule’s severity is an indication of how important it is that this rule matched a query. We use NOTE, WARN, and CRIT to denote these levels.

The rule’s description is a textual, human-readable explanation of what it means when a query matches this rule. Depending on the verbosity of the report you generate, you will see more of the text in the description. By default, you’ll see only the first sentence, which is sort of a terse synopsis of the rule’s meaning. At a higher verbosity, you’ll see subsequent sentences.


severity: note

Aliasing without the AS keyword. Explicitly using the AS keyword in column or table aliases, such as “tbl AS alias,” is more readable than implicit aliases such as “tbl alias”.


severity: warn

Aliasing the ‘*’ wildcard. Aliasing a column wildcard, such as “SELECT tbl.* col1, col2” probably indicates a bug in your SQL. You probably meant for the query to retrieve col1, but instead it renames the last column in the *-wildcarded list.


severity: note

Aliasing without renaming. The table or column’s alias is the same as its real name, and the alias just makes the query harder to read.


severity: warn

Argument with leading wildcard. An argument has a leading wildcard character, such as “%foo”. The predicate with this argument is not sargable and cannot use an index if one exists.


severity: note

LIKE without a wildcard. A LIKE pattern that does not include a wildcard is potentially a bug in the SQL.


severity: warn

SELECT without WHERE. The SELECT statement has no WHERE clause.


severity: note

ORDER BY RAND(). ORDER BY RAND() is a very inefficient way to retrieve a random row from the results.


severity: note

LIMIT with OFFSET. Paginating a result set with LIMIT and OFFSET is O(n^2) complexity, and will cause performance problems as the data grows larger.


severity: note

Ordinal in the GROUP BY clause. Using a number in the GROUP BY clause, instead of an expression or column name, can cause problems if the query is changed.


severity: warn

ORDER BY constant column.


severity: warn

GROUP BY or ORDER BY different tables will force a temp table and filesort.


severity: warn

ORDER BY different directions prevents index from being used. All tables in the ORDER BY clause must be either ASC or DESC, else MySQL cannot use an index.


severity: note

SELECT *. Selecting all columns with the * wildcard will cause the query’s meaning and behavior to change if the table’s schema changes, and might cause the query to retrieve too much data.


severity: note

Blind INSERT. The INSERT or REPLACE query doesn’t specify the columns explicitly, so the query’s behavior will change if the table’s schema changes; use “INSERT INTO tbl(col1, col2) VALUES...” instead.


severity: warn

Storing an IP address as characters. The string literal looks like an IP address, but is not an argument to INET_ATON(), indicating that the data is stored as characters instead of as integers. It is more efficient to store IP addresses as integers.


severity: warn

Unquoted date/time literal. A query such as “WHERE col<2010-02-12” is valid SQL but is probably a bug; the literal should be quoted.


severity: note

SQL_CALC_FOUND_ROWS is inefficient. SQL_CALC_FOUND_ROWS can cause performance problems because it does not scale well; use alternative strategies to build functionality such as paginated result screens.


severity: crit

Mixing comma and ANSI joins. Mixing comma joins and ANSI joins is confusing to humans, and the behavior differs between some MySQL versions.


severity: crit

A table is joined twice. The same table appears at least twice in the FROM clause.


severity: warn

Reference to outer table column in WHERE clause prevents OUTER JOIN, implicitly converts to INNER JOIN.


severity: warn

Exclusion join uses wrong column in WHERE. The exclusion join (LEFT OUTER JOIN with a WHERE clause that is satisfied only if there is no row in the right-hand table) seems to use the wrong column in the WHERE clause. A query such as ”... FROM l LEFT OUTER JOIN r ON l.l=r.r WHERE r.z IS NULL” probably ought to list r.r in the WHERE IS NULL clause.


severity: warn

Non-deterministic GROUP BY. The SQL retrieves columns that are neither in an aggregate function nor the GROUP BY expression, so these values will be non-deterministic in the result.


severity: warn

LIMIT without ORDER BY. LIMIT without ORDER BY causes non-deterministic results, depending on the query execution plan.


severity: note

!= is non-standard. Use the <> operator to test for inequality.


severity: crit

IN() and NOT IN() subqueries are poorly optimized. MySQL executes the subquery as a dependent subquery for each row in the outer query. This is a frequent cause of serious performance problems. This might change version 6.0 of MySQL, but for versions 5.1 and older, the query should be rewritten as a JOIN or a LEFT OUTER JOIN, respectively.


“–query” and “–review” are mutually exclusive.

This tool accepts additional command-line arguments. Refer to the “SYNOPSIS” and usage information for details.

--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.


type: Array

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


default: yes

Continue working even if there is an error.

--daemonize Fork to the background and detach from the shell. POSIX operating systems only.

short form: -D; type: string

Connect to this database. This is also used as the default database for “–[no]show-create-table” if a query does not use database-qualified tables.


short form: -F; type: string

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


type: string; default: rule_id

Group items in the report by this attribute. Possible attributes are:

========= ==========================================================
rule_id   Items matching the same rule ID
query_id  Queries with the same ID (the same fingerprint)
none      No grouping, report each query and its advice individually
--help Show help and exit.

short form: -h; type: string

Connect to host.


type: hash

Ignore these rule IDs.

Specify a comma-separated list of rule IDs (e.g. LIT.001,RES.002,etc.) to ignore. Currently, the rule IDs are case-sensitive and must be uppercase.


short form: -p; type: string

Password to use when connecting.


type: string

Create the given PID file when daemonized. The file contains the process ID of the daemonized instance. The PID file is removed when the daemonized instance exits. The program checks for the existence of the PID file when starting; if it exists and the process with the matching PID exists, the program exits.


short form: -P; type: int

Port number to use for connection.

--print-all Print all queries, even those that do not match any rules. With “–group-by” none, non-matching queries are printed in the main report and profile. For other “–group-by” values, non-matching queries are only printed in the profile. Non-matching queries have zeros for NOTE, WARNand CRIT in the profile.

type: string

Analyze this single query and ignore files and STDIN. This option allows you to supply a single query on the command line. Any files also specified on the command line are ignored.


type: string; default: compact

Type of report format: full or compact. In full mode, every query’s report contains the description of the rules it matched, even if this information was previously displayed. In compact mode, the repeated information is suppressed, and only the rule ID is displayed.


type: DSN

Analyze queries from this pt-query-digest query review table.


type: int; default: 1

How many samples of the query to show.


type: string; default: wait_timeout=10000

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


default: yes

Get SHOW CREATE TABLE for each query’s table.

If host connection options are given (like “–host”, “–port”, etc.) then the tool will also get SHOW CREATE TABLE for each query. This information is needed for some rules like JOI.004. If this option is disabled by specifying --no-show-create-table then some rules may not be checked.


short form: -S; type: string

Socket file to use for connection.


type: Array

The type of input to parse (default slowlog). The permitted types are slowlog and genlog.


short form: -u; type: string

User for login if not current user.


short form: -v; cumulative: yes; default: 1

Increase verbosity of output. At the default level of verbosity, the program prints only the first sentence of each rule’s description. At higher levels, the program prints more of the description. See also “–report-format”.

--version Show version and exit.

type: string

Apply this WHERE clause to the SELECT query on the “–review” table.


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

Database that contains the query review table.

* 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.

* t

Table to use as the query review table.

* 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-query-advisor ... > 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-query-advisor 1.0.2

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