pt-table-usagep - Online in the Cloud

PROGRAM:

NAME

pt-table-usage - Analyze how queries use tables.

SYNOPSIS

Usage: pt-table-usage [OPTIONS] [FILES]

pt-table-usage reads queries from a log and analyzes how they use tables. If no FILE is
specified, it reads STDIN. It prints a report for each query.

RISKS

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

DESCRIPTION

pt-table-usage reads queries from a log and analyzes how they use tables. The log should
be in MySQL's slow query log format.

Table usage is more than simply an indication of which tables the query reads or writes.
It also indicates data flow: data in and data out. The tool determines the data flow by
the contexts in which tables appear. A single query can use a table in several different
contexts simultaneously. The tool's output lists every context for every table. This
CONTEXT-TABLE list indicates how data flows between tables. The "OUTPUT" section lists
the possible contexts and describes how to read a table usage report.

The tool analyzes data flow down to the level of individual columns, so it is helpful if
columns are identified unambiguously in the query. If a query uses only one table, then
all columns must be from that table, and there's no difficulty. But if a query uses
multiple tables and the column names are not table-qualified, then it is necessary to use
"EXPLAIN EXTENDED", followed by "SHOW WARNINGS", to determine to which tables the columns
belong.

If the tool does not know the query's default database, which can occur when the database
is not printed in the log, then "EXPLAIN EXTENDED" can fail. In this case, you can specify
a default database with "--database". You can also use the "--create-table-definitions"
option to help resolve ambiguities.

OUTPUT

The tool prints a usage report for each table in every query, similar to the following:

Query_id: 0x1CD27577D202A339.1
UPDATE t1
SELECT DUAL
JOIN t1
JOIN t2
WHERE t1

Query_id: 0x1CD27577D202A339.2
UPDATE t2
SELECT DUAL
JOIN t1
JOIN t2
WHERE t1

The first line contains the query ID, which by default is the same as those shown in pt-
query-digest reports. It is an MD5 checksum of the query's "fingerprint," which is what
remains after removing literals, collapsing white space, and a variety of other
transformations. The query ID has two parts separated by a period: the query ID and the
table number. If you wish to use a different value to identify the query, you can specify
the "--id-attribute" option.

The previous example shows two paragraphs for a single query, not two queries. Note that
the query ID is identical for the two, but the table number differs. The table number
increments by 1 for each table that the query updates. Only multi-table UPDATE queries
can update multiple tables with a single query, so the table number is 1 for all other
types of queries. (The tool does not support multi-table DELETE queries.) The example
output above is from this query:

UPDATE t1 AS a JOIN t2 AS b USING (id)
SET a.foo="bar", b.foo="bat"
WHERE a.id=1;

The "SET" clause indicates that the query updates two tables: "a" aliased as "t1", and "b"
aliased as "t2".

After the first line, the tool prints a variable number of CONTEXT-TABLE lines. Possible
contexts are as follows:

· SELECT

SELECT means that the query retrieves data from the table for one of two reasons. The
first is to be returned to the user as part of a result set. Only SELECT queries
return result sets, so the report always shows a SELECT context for SELECT queries.

The second case is when data flows to another table as part of an INSERT or UPDATE.
For example, the UPDATE query in the example above has the usage:

SELECT DUAL

This refers to:

SET a.foo="bar", b.foo="bat"

The tool uses DUAL for any values that do not originate in a table, in this case the
literal values "bar" and "bat". If that "SET" clause were "SET a.foo=b.foo" instead,
then the complete usage would be:

Query_id: 0x1CD27577D202A339.1
UPDATE t1
SELECT t2
JOIN t1
JOIN t2
WHERE t1

The presence of a SELECT context after another context, such as UPDATE or INSERT,
indicates where the UPDATE or INSERT retrieves its data. The example immediately
above reflects an UPDATE query that updates rows in table "t1" with data from table
"t2".

· Any other verb

Any other verb, such as INSERT, UPDATE, DELETE, etc. may be a context. These verbs
indicate that the query modifies data in some way. If a SELECT context follows one of
these verbs, then the query reads data from the SELECT table and writes it to this
table. This happens, for example, with INSERT..SELECT or UPDATE queries that use
values from tables instead of constant values.

These query types are not supported: SET, LOAD, and multi-table DELETE.

· JOIN

The JOIN context lists tables that are joined, either with an explicit JOIN in the
FROM clause, or implicitly in the WHERE clause, such as "t1.id = t2.id".

· WHERE

The WHERE context lists tables that are used in the WHERE clause to filter results.
This does not include tables that are implicitly joined in the WHERE clause; those are
listed as JOIN contexts. For example:

WHERE t1.id > 100 AND t1.id < 200 AND t2.foo IS NOT NULL

Results in:

WHERE t1
WHERE t2

The tool lists only distinct tables; that is why table "t1" is listed only once.

· TLIST

The TLIST context lists tables that the query accesses, but which do not appear in any
other context. These tables are usually an implicit cartesian join. For example, the
query "SELECT * FROM t1, t2" results in:

Query_id: 0xBDDEB6EDA41897A8.1
SELECT t1
SELECT t2
TLIST t1
TLIST t2

First of all, there are two SELECT contexts, because "SELECT *" selects rows from all
tables; "t1" and "t2" in this case. Secondly, the tables are implicitly joined, but
without any kind of join condition, which results in a cartesian join as indicated by
the TLIST context for each.

EXIT STATUS

pt-table-usage exits 1 on any kind of error, or 0 if no errors.

OPTIONS

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.

--charset
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.

--config
type: Array

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

--constant-data-value
type: string; default: DUAL

Table to print as the source for constant data (literals). This is any data not
retrieved from tables (or subqueries, because subqueries are not supported). This
includes literal values such as strings ("foo") and numbers (42), or functions such as
"NOW()". For example, in the query "INSERT INTO t (c) VALUES ('a')", the string 'a'
is constant data, so the table usage report is:

INSERT t
SELECT DUAL

The first line indicates that the query inserts data into table "t", and the second
line indicates that the inserted data comes from some constant value.

--[no]continue-on-error
default: yes

Continue to work even if there is an error.

--create-table-definitions
type: array

Read "CREATE TABLE" definitions from this list of comma-separated files. If you
cannot use "--explain-extended" to fully qualify table and column names, you can save
the output of "mysqldump --no-data" to one or more files and specify those files with
this option. The tool will parse all "CREATE TABLE" definitions from the files and
use this information to qualify table and column names. If a column name appears in
multiple tables, or a table name appears in multiple databases, the ambiguities cannot
be resolved.

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

--database
short form: -D; type: string

Default database.

--defaults-file
short form: -F; type: string

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

--explain-extended
type: DSN

A server to execute EXPLAIN EXTENDED queries. This may be necessary to resolve
ambiguous (unqualified) column and table names.

--filter
type: string

Discard events for which this Perl code doesn't return true.

This option is a string of Perl code or a file containing Perl code that is compiled
into a subroutine with one argument: $event. If the given value is a readable file,
then pt-table-usage reads the entire file and uses its contents as the code.

Filters are implemented in the same fashion as in the pt-query-digest tool, so please
refer to its documentation for more information.

--help
Show help and exit.

--host
short form: -h; type: string

Connect to host.

--id-attribute
type: string

Identify each event using this attribute. The default is to use a query ID, which is
an MD5 checksum of the query's fingerprint.

--log
type: string

Print all output to this file when daemonized.

--password
short form: -p; type: string

Password to use when connecting. If password contains commas they must be escaped
with a backslash: "exam\,ple"

--pid
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.

--port
short form: -P; type: int

Port number to use for connection.

--progress
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.

--query
type: string

Analyze the specified query instead of reading a log file.

--read-timeout
type: time; default: 0

Wait this long for an event from the input; 0 to wait forever.

This option sets the maximum time to wait for an event from the input. If an event is
not received after the specified time, the tool stops reading the input and prints its
reports.

This option requires the Perl POSIX module.

--run-time
type: time

How long to run before exiting. The default is to run forever (you can interrupt with
CTRL-C).

--set-vars
type: Array

Set the MySQL variables in this comma-separated list of "variable=value" pairs.

By default, the tool sets:

wait_timeout=10000

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.

--socket
short form: -S; type: string

Socket file to use for connection.

--user
short form: -u; type: string

User for login if not current user.

--version
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 percona-toolkit manpage for full details.

· A

dsn: charset; copy: yes

Default character set.

· D

copy: no

Default database.

· F

dsn: mysql_read_default_file; copy: no

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: no

Socket file to use for connection.

· u

dsn: user; copy: yes

User for login if not current user.

ENVIRONMENT

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-table-usage ... > FILE 2>&1

Be careful: debugging output is voluminous and can generate several megabytes of output.

SYSTEM REQUIREMENTS

You need Perl, DBI, DBD::mysql, and some core packages that ought to be installed in any
reasonably new version of Perl.

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