Provided by: percona-toolkit_3.1+dfsg-1.1_all 
NAME
pt-online-schema-change - ALTER tables without locking them.
SYNOPSIS
Usage: pt-online-schema-change [OPTIONS] DSN
pt-online-schema-change alters a table's structure without blocking reads or writes. Specify the
database and table in the DSN. Do not use this tool before reading its documentation and checking your
backups carefully.
Add a column to sakila.actor:
pt-online-schema-change --alter "ADD COLUMN c1 INT" D=sakila,t=actor
Change sakila.actor to InnoDB, effectively performing OPTIMIZE TABLE in a non-blocking fashion because it
is already an InnoDB table:
pt-online-schema-change --alter "ENGINE=InnoDB" D=sakila,t=actor
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-online-schema-change emulates the way that MySQL alters tables internally, but it works on a copy of
the table you wish to alter. This means that the original table is not locked, and clients may continue
to read and change data in it.
pt-online-schema-change works by creating an empty copy of the table to alter, modifying it as desired,
and then copying rows from the original table into the new table. When the copy is complete, it moves
away the original table and replaces it with the new one. By default, it also drops the original table.
The data copy process is performed in small chunks of data, which are varied to attempt to make them
execute in a specific amount of time (see "--chunk-time"). This process is very similar to how other
tools, such as pt-table-checksum, work. Any modifications to data in the original tables during the copy
will be reflected in the new table, because the tool creates triggers on the original table to update the
corresponding rows in the new table. The use of triggers means that the tool will not work if any
triggers are already defined on the table.
When the tool finishes copying data into the new table, it uses an atomic "RENAME TABLE" operation to
simultaneously rename the original and new tables. After this is complete, the tool drops the original
table.
Foreign keys complicate the tool's operation and introduce additional risk. The technique of atomically
renaming the original and new tables does not work when foreign keys refer to the table. The tool must
update foreign keys to refer to the new table after the schema change is complete. The tool supports two
methods for accomplishing this. You can read more about this in the documentation for
"--alter-foreign-keys-method".
Foreign keys also cause some side effects. The final table will have the same foreign keys and indexes as
the original table (unless you specify differently in your ALTER statement), but the names of the objects
may be changed slightly to avoid object name collisions in MySQL and InnoDB.
For safety, the tool does not modify the table unless you specify the "--execute" option, which is not
enabled by default. The tool supports a variety of other measures to prevent unwanted load or other
problems, including automatically detecting replicas, connecting to them, and using the following safety
checks:
• In most cases the tool will refuse to operate unless a PRIMARY KEY or UNIQUE INDEX is present in the
table. See "--alter" for details.
• The tool refuses to operate if it detects replication filters. See "--[no]check-replication-filters"
for details.
• The tool pauses the data copy operation if it observes any replicas that are delayed in replication.
See "--max-lag" for details.
• The tool pauses or aborts its operation if it detects too much load on the server. See "--max-load"
and "--critical-load" for details.
• The tool sets "innodb_lock_wait_timeout=1" and (for MySQL 5.5 and newer) "lock_wait_timeout=60" so
that it is more likely to be the victim of any lock contention, and less likely to disrupt other
transactions. These values can be changed by specifying "--set-vars".
• The tool refuses to alter the table if foreign key constraints reference it, unless you specify
"--alter-foreign-keys-method".
• The tool cannot alter MyISAM tables on "Percona XtraDB Cluster" nodes.
Percona XtraDB Cluster
pt-online-schema-change works with Percona XtraDB Cluster (PXC) 5.5.28-23.7 and newer, but there are two
limitations: only InnoDB tables can be altered, and "wsrep_OSU_method" must be set to "TOI" (total order
isolation). The tool exits with an error if the host is a cluster node and the table is MyISAM or is
being converted to MyISAM ("ENGINE=MyISAM"), or if "wsrep_OSU_method" is not "TOI". There is no way to
disable these checks.
MySQL 5.7+ Generated columns
The tools ignores MySQL 5.7+ "GENERATED" columns since the value for those columns is generated according
to the expression used to compute column values.
OUTPUT
The tool prints information about its activities to STDOUT so that you can see what it is doing. During
the data copy phase, it prints "--progress" reports to STDERR. You can get additional information by
specifying "--print".
If "--statistics" is specified, a report of various internal event counts is printed at the end, like:
# Event Count
# ====== =====
# INSERT 1
OPTIONS
"--dry-run" and "--execute" are mutually exclusive.
This tool accepts additional command-line arguments. Refer to the "SYNOPSIS" and usage information for
details.
--alter
type: string
The schema modification, without the ALTER TABLE keywords. You can perform multiple modifications to
the table by specifying them with commas. Please refer to the MySQL manual for the syntax of ALTER
TABLE.
The following limitations apply which, if attempted, will cause the tool to fail in unpredictable
ways:
• In almost all cases a PRIMARY KEY or UNIQUE INDEX needs to be present in the table. This is
necessary because the tool creates a DELETE trigger to keep the new table updated while the
process is running.
A notable exception is when a PRIMARY KEY or UNIQUE INDEX is being created from existing columns
as part of the ALTER clause; in that case it will use these column(s) for the DELETE trigger.
• The "RENAME" clause cannot be used to rename the table.
• Columns cannot be renamed by dropping and re-adding with the new name. The tool will not copy
the original column's data to the new column.
• If you add a column without a default value and make it NOT NULL, the tool will fail, as it will
not try to guess a default value for you; You must specify the default.
• "DROP FOREIGN KEY constraint_name" requires specifying "_constraint_name" rather than the real
"constraint_name". Due to a limitation in MySQL, pt-online-schema-change adds a leading
underscore to foreign key constraint names when creating the new table. For example, to drop
this constraint:
CONSTRAINT `fk_foo` FOREIGN KEY (`foo_id`) REFERENCES `bar` (`foo_id`)
You must specify "--alter "DROP FOREIGN KEY _fk_foo"".
• The tool does not use "LOCK IN SHARE MODE" with MySQL 5.0 because it can cause a slave error
which breaks replication:
Query caused different errors on master and slave. Error on master:
'Deadlock found when trying to get lock; try restarting transaction' (1213),
Error on slave: 'no error' (0). Default database: 'pt_osc'.
Query: 'INSERT INTO pt_osc.t (id, c) VALUES ('730', 'new row')'
The error happens when converting a MyISAM table to InnoDB because MyISAM is non-transactional
but InnoDB is transactional. MySQL 5.1 and newer handle this case correctly, but testing
reproduces the error 5% of the time with MySQL 5.0.
This is a MySQL bug, similar to <http://bugs.mysql.com/bug.php?id=45694>, but there is no fix or
workaround in MySQL 5.0. Without "LOCK IN SHARE MODE", tests pass 100% of the time, so the risk
of data loss or breaking replication should be negligible.
Be sure to verify the new table if using MySQL 5.0 and converting from MyISAM to InnoDB!
--alter-foreign-keys-method
type: string
How to modify foreign keys so they reference the new table. Foreign keys that reference the table to
be altered must be treated specially to ensure that they continue to reference the correct table.
When the tool renames the original table to let the new one take its place, the foreign keys "follow"
the renamed table, and must be changed to reference the new table instead.
The tool supports two techniques to achieve this. It automatically finds "child tables" that
reference the table to be altered.
auto
Automatically determine which method is best. The tool uses "rebuild_constraints" if possible
(see the description of that method for details), and if not, then it uses "drop_swap".
rebuild_constraints
This method uses "ALTER TABLE" to drop and re-add foreign key constraints that reference the new
table. This is the preferred technique, unless one or more of the "child" tables is so large
that the "ALTER" would take too long. The tool determines that by comparing the number of rows
in the child table to the rate at which the tool is able to copy rows from the old table to the
new table. If the tool estimates that the child table can be altered in less time than the
"--chunk-time", then it will use this technique. For purposes of estimating the time required to
alter the child table, the tool multiplies the row-copying rate by "--chunk-size-limit", because
MySQL's "ALTER TABLE" is typically much faster than the external process of copying rows.
Due to a limitation in MySQL, foreign keys will not have the same names after the ALTER that they
did prior to it. The tool has to rename the foreign key when it redefines it, which adds a
leading underscore to the name. In some cases, MySQL also automatically renames indexes required
for the foreign key.
drop_swap
Disable foreign key checks (FOREIGN_KEY_CHECKS=0), then drop the original table before renaming
the new table into its place. This is different from the normal method of swapping the old and
new table, which uses an atomic "RENAME" that is undetectable to client applications.
This method is faster and does not block, but it is riskier for two reasons. First, for a short
time between dropping the original table and renaming the temporary table, the table to be
altered simply does not exist, and queries against it will result in an error. Secondly, if
there is an error and the new table cannot be renamed into the place of the old one, then it is
too late to abort, because the old table is gone permanently.
This method forces "--no-swap-tables" and "--no-drop-old-table".
none
This method is like "drop_swap" without the "swap". Any foreign keys that referenced the
original table will now reference a nonexistent table. This will typically cause foreign key
violations that are visible in "SHOW ENGINE INNODB STATUS", similar to the following:
Trying to add to index `idx_fk_staff_id` tuple:
DATA TUPLE: 2 fields;
0: len 1; hex 05; asc ;;
1: len 4; hex 80000001; asc ;;
But the parent table `sakila`.`staff_old`
or its .ibd file does not currently exist!
This is because the original table (in this case, sakila.staff) was renamed to sakila.staff_old
and then dropped. This method of handling foreign key constraints is provided so that the
database administrator can disable the tool's built-in functionality if desired.
--[no]analyze-before-swap
default: yes
Execute ANALYZE TABLE on the new table before swapping with the old one. By default, this happens
only when running MySQL 5.6 and newer, and "innodb_stats_persistent" is enabled. Specify the option
explicitly to enable or disable it regardless of MySQL version and "innodb_stats_persistent".
This circumvents a potentially serious issue related to InnoDB optimizer statistics. If the table
being alerted is busy and the tool completes quickly, the new table will not have optimizer
statistics after being swapped. This can cause fast, index-using queries to do full table scans until
optimizer statistics are updated (usually after 10 seconds). If the table is large and the server
very busy, this can cause an outage.
--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.
--[no]check-alter
default: yes
Parses the "--alter" specified and tries to warn of possible unintended behavior. Currently, it
checks for:
Column renames
In previous versions of the tool, renaming a column with "CHANGE COLUMN name new_name" would lead
to that column's data being lost. The tool now parses the alter statement and tries to catch
these cases, so the renamed columns should have the same data as the originals. However, the code
that does this is not a full-blown SQL parser, so you should first run the tool with "--dry-run"
and "--print" and verify that it detects the renamed columns correctly.
DROP PRIMARY KEY
If "--alter" contain "DROP PRIMARY KEY" (case- and space-insensitive), a warning is printed and
the tool exits unless "--dry-run" is specified. Altering the primary key can be dangerous, but
the tool can handle it. The tool's triggers, particularly the DELETE trigger, are most affected
by altering the primary key because the tool prefers to use the primary key for its triggers.
You should first run the tool with "--dry-run" and "--print" and verify that the triggers are
correct.
--check-interval
type: time; default: 1
Sleep time between checks for "--max-lag".
--[no]check-plan
default: yes
Check query execution plans for safety. By default, this option causes the tool to run EXPLAIN before
running queries that are meant to access a small amount of data, but which could access many rows if
MySQL chooses a bad execution plan. These include the queries to determine chunk boundaries and the
chunk queries themselves. If it appears that MySQL will use a bad query execution plan, the tool will
skip the chunk of the table.
The tool uses several heuristics to determine whether an execution plan is bad. The first is whether
EXPLAIN reports that MySQL intends to use the desired index to access the rows. If MySQL chooses a
different index, the tool considers the query unsafe.
The tool also checks how much of the index MySQL reports that it will use for the query. The EXPLAIN
output shows this in the key_len column. The tool remembers the largest key_len seen, and skips
chunks where MySQL reports that it will use a smaller prefix of the index. This heuristic can be
understood as skipping chunks that have a worse execution plan than other chunks.
The tool prints a warning the first time a chunk is skipped due to a bad execution plan in each
table. Subsequent chunks are skipped silently, although you can see the count of skipped chunks in
the SKIPPED column in the tool's output.
This option adds some setup work to each table and chunk. Although the work is not intrusive for
MySQL, it results in more round-trips to the server, which consumes time. Making chunks too small
will cause the overhead to become relatively larger. It is therefore recommended that you not make
chunks too small, because the tool may take a very long time to complete if you do.
--[no]check-replication-filters
default: yes
Abort if any replication filter is set on any server. The tool looks for server options that filter
replication, such as binlog_ignore_db and replicate_do_db. If it finds any such filters, it aborts
with an error.
If the replicas are configured with any filtering options, you should be careful not to modify any
databases or tables that exist on the master and not the replicas, because it could cause replication
to fail. For more information on replication rules, see
<http://dev.mysql.com/doc/en/replication-rules.html>.
--check-slave-lag
type: string
Pause the data copy until this replica's lag is less than "--max-lag". The value is a DSN that
inherits properties from the the connection options ("--port", "--user", etc.). This option
overrides the normal behavior of finding and continually monitoring replication lag on ALL connected
replicas. If you don't want to monitor ALL replicas, but you want more than just one replica to be
monitored, then use the DSN option to the "--recursion-method" option instead of this option.
--chunk-index
type: string
Prefer this index for chunking tables. By default, the tool chooses the most appropriate index for
chunking. This option lets you specify the index that you prefer. If the index doesn't exist, then
the tool will fall back to its default behavior of choosing an index. The tool adds the index to the
SQL statements in a "FORCE INDEX" clause. Be careful when using this option; a poor choice of index
could cause bad performance.
--chunk-index-columns
type: int
Use only this many left-most columns of a "--chunk-index". This works only for compound indexes, and
is useful in cases where a bug in the MySQL query optimizer (planner) causes it to scan a large range
of rows instead of using the index to locate starting and ending points precisely. This problem
sometimes occurs on indexes with many columns, such as 4 or more. If this happens, the tool might
print a warning related to the "--[no]check-plan" option. Instructing the tool to use only the first
N columns of the index is a workaround for the bug in some cases.
--chunk-size
type: size; default: 1000
Number of rows to select for each chunk copied. Allowable suffixes are k, M, G.
This option can override the default behavior, which is to adjust chunk size dynamically to try to
make chunks run in exactly "--chunk-time" seconds. When this option isn't set explicitly, its
default value is used as a starting point, but after that, the tool ignores this option's value. If
you set this option explicitly, however, then it disables the dynamic adjustment behavior and tries
to make all chunks exactly the specified number of rows.
There is a subtlety: if the chunk index is not unique, then it's possible that chunks will be larger
than desired. For example, if a table is chunked by an index that contains 10,000 of a given value,
there is no way to write a WHERE clause that matches only 1,000 of the values, and that chunk will be
at least 10,000 rows large. Such a chunk will probably be skipped because of "--chunk-size-limit".
--chunk-size-limit
type: float; default: 4.0
Do not copy chunks this much larger than the desired chunk size.
When a table has no unique indexes, chunk sizes can be inaccurate. This option specifies a maximum
tolerable limit to the inaccuracy. The tool uses <EXPLAIN> to estimate how many rows are in the
chunk. If that estimate exceeds the desired chunk size times the limit, then the tool skips the
chunk.
The minimum value for this option is 1, which means that no chunk can be larger than "--chunk-size".
You probably don't want to specify 1, because rows reported by EXPLAIN are estimates, which can be
different from the real number of rows in the chunk. You can disable oversized chunk checking by
specifying a value of 0.
The tool also uses this option to determine how to handle foreign keys that reference the table to be
altered. See "--alter-foreign-keys-method" for details.
--chunk-time
type: float; default: 0.5
Adjust the chunk size dynamically so each data-copy query takes this long to execute. The tool
tracks the copy rate (rows per second) and adjusts the chunk size after each data-copy query, so that
the next query takes this amount of time (in seconds) to execute. It keeps an exponentially decaying
moving average of queries per second, so that if the server's performance changes due to changes in
server load, the tool adapts quickly.
If this option is set to zero, the chunk size doesn't auto-adjust, so query times will vary, but
query chunk sizes will not. Another way to do the same thing is to specify a value for "--chunk-size"
explicitly, instead of leaving it at the default.
--config
type: Array
Read this comma-separated list of config files; if specified, this must be the first option on the
command line.
--critical-load
type: Array; default: Threads_running=50
Examine SHOW GLOBAL STATUS after every chunk, and abort if the load is too high. The option accepts
a comma-separated list of MySQL status variables and thresholds. An optional "=MAX_VALUE" (or
":MAX_VALUE") can follow each variable. If not given, the tool determines a threshold by examining
the current value at startup and doubling it.
See "--max-load" for further details. These options work similarly, except that this option will
abort the tool's operation instead of pausing it, and the default value is computed differently if
you specify no threshold. The reason for this option is as a safety check in case the triggers on
the original table add so much load to the server that it causes downtime. There is probably no
single value of Threads_running that is wrong for every server, but a default of 50 seems likely to
be unacceptably high for most servers, indicating that the operation should be canceled immediately.
--database
short form: -D; type: string
Connect to this database.
--default-engine
Remove "ENGINE" from the new table.
By default the new table is created with the same table options as the original table, so if the
original table uses InnoDB, then the new table will use InnoDB. In certain cases involving
replication, this may cause unintended changes on replicas which use a different engine for the same
table. Specifying this option causes the new table to be created with the system's default engine.
--data-dir
type: string
Create the new table on a different partition using the DATA DIRECTORY feature. Only available on
5.6+. This parameter is ignored if it is used at the same time than remove-data-dir.
--remove-data-dir
default: no
If the original table was created using the DATA DIRECTORY feature, remove it and create the new
table in MySQL default directory without creating a new isl file.
--defaults-file
short form: -F; type: string
Only read mysql options from the given file. You must give an absolute pathname.
--[no]drop-new-table
default: yes
Drop the new table if copying the original table fails.
Specifying "--no-drop-new-table" and "--no-swap-tables" leaves the new, altered copy of the table
without modifying the original table. See "--new-table-name".
--no-drop-new-table does not work with "alter-foreign-keys-method drop_swap".
--[no]drop-old-table
default: yes
Drop the original table after renaming it. After the original table has been successfully renamed to
let the new table take its place, and if there are no errors, the tool drops the original table by
default. If there are any errors, the tool leaves the original table in place.
If "--no-swap-tables" is specified, then there is no old table to drop.
--[no]drop-triggers
default: yes
Drop triggers on the old table. "--no-drop-triggers" forces "--no-drop-old-table".
--dry-run
Create and alter the new table, but do not create triggers, copy data, or replace the original table.
--execute
Indicate that you have read the documentation and want to alter the table. You must specify this
option to alter the table. If you do not, then the tool will only perform some safety checks and
exit. This helps ensure that you have read the documentation and understand how to use this tool.
If you have not read the documentation, then do not specify this option.
--[no]check-unique-key-change
default: yes
Avoid "pt-online-schema-change" to run if the specified statement for "--alter" is trying to add an
unique index. Since "pt-online-schema-change" uses "INSERT IGNORE" to copy rows to the new table, if
the row being written produces a duplicate key, it will fail silently and data will be lost.
Example:
CREATE DATABASE test;
USE test;
CREATE TABLE `a` (
`id` int(11) NOT NULL,
`unique_id` varchar(32) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
insert into a values (1, "a");
insert into a values (2, "b");
insert into a values (3, "");
insert into a values (4, "");
insert into a values (5, NULL);
insert into a values (6, NULL);
Using "pt-online-schema-change" to add an unique index on the "unique_id" field, will cause some rows
to be lost due to the use of "INSERT IGNORE" to copy rows from the source table. For this reason,
"pt-online-schema-change" will fail if it detects that the "--alter" parameter is trying to add an
unique key and it will show an example query to run to detect if there are rows that will produce
duplicated indexes.
Even if you run the query and there are no rows that will produce duplicated indexes, take into
consideration that after running this query, changes can be made to the table that can produce
duplicate rows and this data will be lost.
--force
This options bypasses confirmation in case of using alter-foreign-keys-method = none , which might
break foreign key constraints.
--force-concat-enums
The NibbleIterator in Percona Toolkit can detect indexes having ENUM fields and if the items it has
are sorted or not. According to MySQL documentation at
<https://dev.mysql.com/doc/refman/8.0/en/enum.html>:
ENUM values are sorted based on their index numbers, which depend on the order in which the
enumeration members were listed in the column specification. For example, 'b' sorts before 'a' for
ENUM('b', 'a'). The empty string sorts before nonempty strings, and NULL values sort before all
other enumeration values.
To prevent unexpected results when using the ORDER BY clause on an ENUM column, use one of these
techniques: - Specify the ENUM list in alphabetic order. - Make sure that the column is sorted
lexically rather than by index number by coding ORDER BY CAST(col AS CHAR) or ORDER BY CONCAT(col).
The NibbleIterator in Percona Toolkit uses CONCAT(col) but, doing that, adds overhead since MySQL
cannot use the column directly and has to calculate the result of CONCAT for every row. To make this
scenario vissible to the user, if there are indexes having ENUM fields with usorted items, it is
necessary to specify the "--force-concat-enums" parameter.
--help
Show help and exit.
--host
short form: -h; type: string
Connect to host.
--max-flow-ctl
type: float
Somewhat similar to --max-lag but for PXC clusters. Check average time cluster spent pausing for
Flow Control and make tool pause if it goes over the percentage indicated in the option. A value of
0 would make the tool pause when *any* Flow Control activity is detected. Default is no Flow Control
checking. This option is available for PXC versions 5.6 or higher.
--max-lag
type: time; default: 1s
Pause the data copy until all replicas' lag is less than this value. After each data-copy query
(each chunk), the tool looks at the replication lag of all replicas to which it connects, using
Seconds_Behind_Master. If any replica is lagging more than the value of this option, then the tool
will sleep for "--check-interval" seconds, then check all replicas again. If you specify
"--check-slave-lag", then the tool only examines that server for lag, not all servers. If you want
to control exactly which servers the tool monitors, use the DSN value to "--recursion-method".
The tool waits forever for replicas to stop lagging. If any replica is stopped, the tool waits
forever until the replica is started. The data copy continues when all replicas are running and not
lagging too much.
The tool prints progress reports while waiting. If a replica is stopped, it prints a progress report
immediately, then again at every progress report interval.
--max-load
type: Array; default: Threads_running=25
Examine SHOW GLOBAL STATUS after every chunk, and pause if any status variables are higher than their
thresholds. The option accepts a comma-separated list of MySQL status variables. An optional
"=MAX_VALUE" (or ":MAX_VALUE") can follow each variable. If not given, the tool determines a
threshold by examining the current value and increasing it by 20%.
For example, if you want the tool to pause when Threads_connected gets too high, you can specify
"Threads_connected", and the tool will check the current value when it starts working and add 20% to
that value. If the current value is 100, then the tool will pause when Threads_connected exceeds
120, and resume working when it is below 120 again. If you want to specify an explicit threshold,
such as 110, you can use either "Threads_connected:110" or "Threads_connected=110".
The purpose of this option is to prevent the tool from adding too much load to the server. If the
data-copy queries are intrusive, or if they cause lock waits, then other queries on the server will
tend to block and queue. This will typically cause Threads_running to increase, and the tool can
detect that by running SHOW GLOBAL STATUS immediately after each query finishes. If you specify a
threshold for this variable, then you can instruct the tool to wait until queries are running
normally again. This will not prevent queueing, however; it will only give the server a chance to
recover from the queueing. If you notice queueing, it is best to decrease the chunk time.
--preserve-triggers
Preserves old triggers when specified. As of MySQL 5.7.2, it is possible to define multiple triggers
for a given table that have the same trigger event and action time. This allows us to add the
triggers needed for "pt-online-schema-change" even if the table already has its own triggers. If
this option is enabled, "pt-online-schema-change" will try to copy all the existing triggers to the
new table BEFORE start copying rows from the original table to ensure the old triggers can be applied
after altering the table.
Example.
CREATE TABLE test.t1 (
id INT NOT NULL AUTO_INCREMENT,
f1 INT,
f2 VARCHAR(32),
PRIMARY KEY (id)
);
CREATE TABLE test.log (
ts TIMESTAMP,
msg VARCHAR(255)
);
CREATE TRIGGER test.after_update
AFTER
UPDATE ON test.t1
FOR EACH ROW
INSERT INTO test.log VALUES (NOW(), CONCAT("updated row row with id ", OLD.id, " old f1:", OLD.f1, " new f1: ", NEW.f1 ));
For this table and triggers combination, it is not possible to use --preserve-triggers with an
--alter like this: "DROP COLUMN f1" since the trigger references the column being dropped and at
would make the trigger to fail.
After testing the triggers will work on the new table, the triggers are dropped from the new table
until all rows have been copied and then they are re-applied.
--preserve-triggers cannot be used with these other parameters, --no-drop-triggers,
--no-drop-old-table and --no-swap-tables since --preserve-triggers implies that the old triggers
should be deleted and recreated in the new table. Since it is not possible to have more than one
trigger with the same name, old triggers must be deleted in order to be able to recreate them into
the new table.
Using "--preserve-triggers" with "--no-swap-tables" will cause triggers to remain defined for the
original table. Please read the documentation for --swap-tables
If both "--no-swap-tables" and "--no-drop-new-table" is set, the trigger will remain on the original
table and will be duplicated on the new table (the trigger will have a random suffix as no trigger
names are unique).
--new-table-name
type: string; default: %T_new
New table name before it is swapped. %T is replaced with the original table name. When the default
is used, the tool prefixes the name with up to 10 "_" (underscore) to find a unique table name. If a
table name is specified, the tool does not prefix it with "_", so the table must not exist.
--null-to-not-null
Allows MODIFYing a column that allows NULL values to one that doesn't allow them. The rows which
contain NULL values will be converted to the defined default value. If no explicit DEFAULT value is
given MySQL will assign a default value based on datatype, e.g. 0 for number datatypes, '' for string
datatypes.
--only-same-schema-fks
Check foreigns keys only on tables on the same schema than the original table. This option is
dangerous since if you have FKs refenrencing tables in other schemas, they won't be detected.
--password
short form: -p; type: string
Password to use when connecting. If password contains commas they must be escaped with a backslash:
"exam\,ple"
--pause-file
type: string
Execution will be paused while the file specified by this param exists.
--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.
--plugin
type: string
Perl module file that defines a "pt_online_schema_change_plugin" class. A plugin allows you to write
a Perl module that can hook into many parts of pt-online-schema-change. This requires a good
knowledge of Perl and Percona Toolkit conventions, which are beyond this scope of this documentation.
Please contact Percona if you have questions or need help.
See "PLUGIN" for more information.
--port
short form: -P; type: int
Port number to use for connection.
--print
Print SQL statements to STDOUT. Specifying this option allows you to see most of the statements that
the tool executes. You can use this option with "--dry-run", for example.
--progress
type: array; default: time,30
Print progress reports to STDERR while copying rows. 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.
--quiet
short form: -q
Do not print messages to STDOUT (disables "--progress"). Errors and warnings are still printed to
STDERR.
--recurse
type: int
Number of levels to recurse in the hierarchy when discovering replicas. Default is infinite. See
also "--recursion-method".
--recursion-method
type: array; default: processlist,hosts
Preferred recursion method for discovering replicas. Possible methods are:
METHOD USES
=========== ==================
processlist SHOW PROCESSLIST
hosts SHOW SLAVE HOSTS
dsn=DSN DSNs from a table
none Do not find slaves
The processlist method is the default, because SHOW SLAVE HOSTS is not reliable. However, the hosts
method can work better if the server uses a non-standard port (not 3306). The tool usually does the
right thing and finds all replicas, but you may give a preferred method and it will be used first.
The hosts method requires replicas to be configured with report_host, report_port, etc.
The dsn method is special: it specifies a table from which other DSN strings are read. The specified
DSN must specify a D and t, or a database-qualified t. The DSN table should have the following
structure:
CREATE TABLE `dsns` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`parent_id` int(11) DEFAULT NULL,
`dsn` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
);
To make the tool monitor only the hosts 10.10.1.16 and 10.10.1.17 for replication lag, insert the
values "h=10.10.1.16" and "h=10.10.1.17" into the table. Currently, the DSNs are ordered by id, but
id and parent_id are otherwise ignored.
You can change the list of hosts while OSC is executing: if you change the contents of the DSN table,
OSC will pick it up very soon.
--skip-check-slave-lag
type: DSN; repeatable: yes
DSN to skip when checking slave lag. It can be used multiple times. Example: --skip-check-slave-lag
h=127.0.0.1,P=12345 --skip-check-slave-lag h=127.0.0.1,P=12346 Please take into consideration that
even when for the MySQL driver h=127.1 is equal to h=127.0.0.1, for this parameter you need to
specify the full IP address.
--slave-user
type: string
Sets the user to be used to connect to the slaves. This parameter allows you to have a different
user with less privileges on the slaves but that user must exist on all slaves.
--slave-password
type: string
Sets the password to be used to connect to the slaves. It can be used with --slave-user and the
password for the user must be the same on all slaves.
--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
innodb_lock_wait_timeout=1
lock_wait_timeout=60
Variables specified on the command line override these defaults. For example, specifying "--set-vars
wait_timeout=500" overrides the default value of 10000.
The tool prints a warning and continues if a variable cannot be set.
Note that setting the "sql_mode" variable requires some tricky escapes to be able to parse the quotes
and commas.
Example:
--set-vars sql_mode=\'STRICT_ALL_TABLES\\,ALLOW_INVALID_DATES\'
Note the single backslash for the quotes and double backslash for the comma.
--sleep
type: float; default: 0
How long to sleep (in seconds) after copying each chunk. This option is useful when throttling by
"--max-lag" and "--max-load" are not possible. A small, sub-second value should be used, like 0.1,
else the tool could take a very long time to copy large tables.
--socket
short form: -S; type: string
Socket file to use for connection.
--statistics
Print statistics about internal counters. This is useful to see how many warnings were suppressed
compared to the number of INSERT.
--[no]swap-tables
default: yes
Swap the original table and the new, altered table. This step completes the online schema change
process by making the table with the new schema take the place of the original table. The original
table becomes the "old table," and the tool drops it unless you disable "--[no]drop-old-table".
Using "--no-swap-tables" will run the whole process, it will create the new table, it will copy all
rows but at the end it will drop the new table. It is intended to run a more realistic --dry-run.
--tries
type: array
How many times to try critical operations. If certain operations fail due to non-fatal, recoverable
errors, the tool waits and tries the operation again. These are the operations that are retried,
with their default number of tries and wait time between tries (in seconds):
OPERATION TRIES WAIT
=================== ===== ====
create_triggers 10 1
drop_triggers 10 1
copy_rows 10 0.25
swap_tables 10 1
update_foreign_keys 10 1
analyze_table 10 1
To change the defaults, specify the new values like:
--tries create_triggers:5:0.5,drop_triggers:5:0.5
That makes the tool try "create_triggers" and "drop_triggers" 5 times with a 0.5 second wait between
tries. So the format is:
operation:tries:wait[,operation:tries:wait]
All three values must be specified.
Note that most operations are affected only in MySQL 5.5 and newer by "lock_wait_timeout" (see
"--set-vars") because of metadata locks. The "copy_rows" operation is affected in any version of
MySQL by "innodb_lock_wait_timeout".
For creating and dropping triggers, the number of tries applies to each "CREATE TRIGGER" and "DROP
TRIGGER" statement for each trigger. For copying rows, the number of tries applies to each chunk,
not the entire table. For swapping tables, the number of tries usually applies once because there is
usually only one "RENAME TABLE" statement. For rebuilding foreign key constraints, the number of
tries applies to each statement ("ALTER" statements for the "rebuild_constraints"
"--alter-foreign-keys-method"; other statements for the "drop_swap" method).
The tool retries each operation if these errors occur:
Lock wait timeout (innodb_lock_wait_timeout and lock_wait_timeout)
Deadlock found
Query is killed (KILL QUERY <thread_id>)
Connection is killed (KILL CONNECTION <thread_id>)
Lost connection to MySQL
In the case of lost and killed connections, the tool will automatically reconnect.
Failures and retries are recorded in the "--statistics".
--user
short form: -u; type: string
User for login if not current user.
--version
Show version and exit.
--[no]version-check
default: yes
Check for the latest version of Percona Toolkit, MySQL, and other programs.
This is a standard "check for updates automatically" feature, with two additional features. First,
the tool checks its own version and also the versions of the following software: operating system,
Percona Monitoring and Management (PMM), MySQL, Perl, MySQL driver for Perl (DBD::mysql), and Percona
Toolkit. 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.
A secure connection to Percona’s Version Check database server is done to perform these checks. Each
request is logged by the server, including software version numbers and unique ID of the checked
system. The ID is generated by the Percona Toolkit installation script or when the Version Check
database call is done for the first time.
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 <https://www.percona.com/doc/percona-toolkit/LATEST/version-check.html>.
PLUGIN
The file specified by "--plugin" must define a class (i.e. a package) called
"pt_online_schema_change_plugin" with a "new()" subroutine. The tool will create an instance of this
class and call any hooks that it defines. No hooks are required, but a plugin isn't very useful without
them.
These hooks, in this order, are called if defined:
init
before_create_new_table
after_create_new_table
before_alter_new_table
after_alter_new_table
before_create_triggers
after_create_triggers
before_copy_rows
after_copy_rows
before_swap_tables
after_swap_tables
before_update_foreign_keys
after_update_foreign_keys
before_drop_old_table
after_drop_old_table
before_drop_triggers
before_exit
get_slave_lag
Each hook is passed different arguments. To see which arguments are passed to a hook, search for the
hook's name in the tool's source code, like:
# --plugin hook
if ( $plugin && $plugin->can('init') ) {
$plugin->init(
orig_tbl => $orig_tbl,
child_tables => $child_tables,
renamed_cols => $renamed_cols,
slaves => $slaves,
slave_lag_cxns => $slave_lag_cxns,
);
}
The comment "# --plugin hook" precedes every hook call.
Here's a plugin file template for all hooks:
package pt_online_schema_change_plugin;
use strict;
sub new {
my ($class, %args) = @_;
my $self = { %args };
return bless $self, $class;
}
sub init {
my ($self, %args) = @_;
print "PLUGIN init\n";
}
sub before_create_new_table {
my ($self, %args) = @_;
print "PLUGIN before_create_new_table\n";
}
sub after_create_new_table {
my ($self, %args) = @_;
print "PLUGIN after_create_new_table\n";
}
sub before_alter_new_table {
my ($self, %args) = @_;
print "PLUGIN before_alter_new_table\n";
}
sub after_alter_new_table {
my ($self, %args) = @_;
print "PLUGIN after_alter_new_table\n";
}
sub before_create_triggers {
my ($self, %args) = @_;
print "PLUGIN before_create_triggers\n";
}
sub after_create_triggers {
my ($self, %args) = @_;
print "PLUGIN after_create_triggers\n";
}
sub before_copy_rows {
my ($self, %args) = @_;
print "PLUGIN before_copy_rows\n";
}
sub after_copy_rows {
my ($self, %args) = @_;
print "PLUGIN after_copy_rows\n";
}
sub before_swap_tables {
my ($self, %args) = @_;
print "PLUGIN before_swap_tables\n";
}
sub after_swap_tables {
my ($self, %args) = @_;
print "PLUGIN after_swap_tables\n";
}
sub before_update_foreign_keys {
my ($self, %args) = @_;
print "PLUGIN before_update_foreign_keys\n";
}
sub after_update_foreign_keys {
my ($self, %args) = @_;
print "PLUGIN after_update_foreign_keys\n";
}
sub before_drop_old_table {
my ($self, %args) = @_;
print "PLUGIN before_drop_old_table\n";
}
sub after_drop_old_table {
my ($self, %args) = @_;
print "PLUGIN after_drop_old_table\n";
}
sub before_drop_triggers {
my ($self, %args) = @_;
print "PLUGIN before_drop_triggers\n";
}
sub before_exit {
my ($self, %args) = @_;
print "PLUGIN before_exit\n";
}
sub get_slave_lag {
my ($self, %args) = @_;
print "PLUGIN get_slave_lag\n";
return sub { return 0; };
}
1;
Notice that "get_slave_lag" must return a function reference; ideally one that returns actual slave lag,
not simply zero like in the example.
Here's an example that actually does something:
package pt_online_schema_change_plugin;
use strict;
sub new {
my ($class, %args) = @_;
my $self = { %args };
return bless $self, $class;
}
sub after_create_new_table {
my ($self, %args) = @_;
my $new_tbl = $args{new_tbl};
my $dbh = $self->{cxn}->dbh;
my $row = $dbh->selectrow_arrayref("SHOW CREATE TABLE $new_tbl->{name}");
warn "after_create_new_table: $row->[1]\n\n";
}
sub after_alter_new_table {
my ($self, %args) = @_;
my $new_tbl = $args{new_tbl};
my $dbh = $self->{cxn}->dbh;
my $row = $dbh->selectrow_arrayref("SHOW CREATE TABLE $new_tbl->{name}");
warn "after_alter_new_table: $row->[1]\n\n";
}
1;
You could use this with "--dry-run" to check how the table will look before and after.
Please contact Percona if you have questions or need help.
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
dsn: database; copy: no
Database for the old and new 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. 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.
• t
dsn: table; copy: no
Table to alter.
• 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-online-schema-change ... > FILE 2>&1
Be careful: debugging output is voluminous and can generate several megabytes of output.
EXIT STATUS
INVALID_PARAMETERS = 1
UNSUPORTED_MYSQL_VERSION = 2
NO_MINIMUM_REQUIREMENTS = 3
NO_PRIMARY_OR_UNIQUE_KEY = 4
INVALID_PLUGIN_FILE = 5
INVALID_ALTER_FK_METHOD = 6
INVALID_KEY_SIZE = 7
CANNOT_DETERMINE_KEY_SIZE = 9
NOT_SAFE_TO_ASCEND = 9
ERROR_CREATING_NEW_TABLE = 10
ERROR_ALTERING_TABLE = 11
ERROR_CREATING_TRIGGERS = 12
ERROR_RESTORING_TRIGGERS = 13
ERROR_SWAPPING_TABLES = 14
ERROR_UPDATING_FKS = 15
ERROR_DROPPING_OLD_TABLE = 16
UNSUPORTED_OPERATION = 17
MYSQL_CONNECTION_ERROR = 18
LOST_MYSQL_CONNECTION = 19
SYSTEM REQUIREMENTS
You need Perl, DBI, DBD::mysql, and some core packages that ought to be installed in any reasonably new
version of Perl.
This tool works only on MySQL 5.0.2 and newer versions, because earlier versions do not support triggers.
Also a number of permissions should be set on MySQL to make pt-online-schema-change operate as expected.
PROCESS, SUPER, REPLICATION SLAVE global privileges, as well as SELECT, INSERT, UPDATE, DELETE, CREATE,
DROP, ALTER, and TRIGGER table privileges should be granted on server. Slave needs only REPLICATION SLAVE
and REPLICATION CLIENT privileges.
BUGS
For a list of known bugs, see <http://www.percona.com/bugs/pt-online-schema-change>.
Please report bugs at <https://bugs.launchpad.net/percona-toolkit>. 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".
DOWNLOADING
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.
AUTHORS
Daniel Nichter and Baron Schwartz
ACKNOWLEDGMENTS
The "online schema change" concept was first implemented by Shlomi Noach in his tool
"oak-online-alter-table", part of <http://code.google.com/p/openarkkit/>. Engineers at Facebook then
built another version called "OnlineSchemaChange.php" as explained by their blog post:
<http://tinyurl.com/32zeb86>. This tool is a hybrid of both approaches, with additional features and
functionality not present in either.
ABOUT PERCONA TOOLKIT
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.
COPYRIGHT, LICENSE, AND WARRANTY
This program is copyright 2011-2018 Percona LLC and/or its affiliates.
THIS PROGRAM IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT
LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.
VERSION
pt-online-schema-change 3.1.0
POD ERRORS
Hey! The above document had some coding errors, which are explained below:
Around line 12791:
Non-ASCII character seen before =encoding in 'Percona’s'. Assuming UTF-8