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pt-online-schema-change - ALTER tables without locking them.



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

pt-online-schema-change --alter "ADD COLUMN c1 INT" D=sakila,t=actor

Change 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


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


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.

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.


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


--dry-run and --execute are mutually exclusive.

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


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, 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!


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.


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.


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.


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.


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.


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.


Prompt for a password when connecting to MySQL.


type: string

Channel name used when connected to a server using replication channels. Suppose you have two masters, master_a at port 12345, master_b at port 1236 and a slave connected to both masters using channels chan_master_a and chan_master_b. If you want to run pt-table-sync to synchronize the slave against master_a, pt-table-sync won’t be able to determine what’s the correct master since SHOW SLAVE STATUS will return 2 rows. In this case, you can use –channel=chan_master_a to specify the channel name to use in the SHOW SLAVE STATUS command.


short form: -A; type: string

Default character set. If the value is utf8, sets Perl’s binmode on STDOUT to utf8, passes the mysql_enable_utf8 option to DBD::mysql, and runs SET NAMES UTF8 after connecting to MySQL. Any other value sets binmode on STDOUT without the utf8 layer, and runs SET NAMES after connecting to MySQL.


default: yes

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.


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.


default: yes

Check for self-referencing foreign keys. Currently self referencing FKs are not full supported, so, to prevent errors, this program won’t run if the table has self-referencing foreign keys. Use this parameter to disable self-referencing FK checks.


type: time; default: 1

Sleep time between checks for --max-lag.


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 stop copying rows and die.

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 terminates if MySQL reports that it will use a smaller prefix of the index.

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.


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


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.


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.


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.


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.


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 will stop copying rows and die.

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.


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.


type: Array

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


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.


short form: -D; type: string

Connect to this database.


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.


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.


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.


short form: -F; type: string

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


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.


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.


default: yes

Drop triggers on the old table. --no-drop-triggers forces --no-drop-old-table.


Create and alter the new table, but do not create triggers, copy data, or replace the original table.


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.


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.


USE test;
  `id` int(11) NOT NULL,
  `unique_id` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)

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.


This options bypasses confirmation in case of using alter-foreign-keys-method = none , which might break foreign key constraints.


Show help and exit.


short form: -h; type: string

Connect to host.


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.


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.


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.


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.


CREATE TABLE test.t1 (
     f1 INT,
     f2 VARCHAR(32),
     PRIMARY KEY (id)

CREATE TABLE test.log (
   msg VARCHAR(255)

CREATE TRIGGER test.after_update
   UPDATE ON test.t1
     INSERT INTO test.log VALUES (NOW(), CONCAT("updated row row with 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).


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.


Allows MODIFYing a column that allows NULL values to one that doesn’t allow them. The existing rows which contain NULL values will be converted to the default value based on datatype, e.g. 0 for number datatypes, ‘’ for string datatypes. New rows will use the user defined default value if specified for the column.


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.


short form: -p; type: string

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


type: string

Execution will be paused while the file specified by this param exists.


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.


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.


short form: -P; type: int

Port number to use for connection.


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.


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.


short form: -q

Do not print messages to STDOUT (disables --progress). Errors and warnings are still printed to STDERR.


type: int

Number of levels to recurse in the hierarchy when discovering replicas. Default is infinite. See also --recursion-method.


type: array; default: processlist,hosts

Preferred recursion method for discovering replicas. Possible methods are:

===========  ==================
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:

  `parent_id` int(11) DEFAULT NULL,
  `dsn` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)

To make the tool monitor only the hosts and for replication lag, insert the values h= and h= 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.

–reverse-triggers Copy the triggers added during the copy in reverse order. Commands in the new table will be reflected in the old table. You can use this as a safety feature, so that the old table continues to receive updates. This option requires --no-drop-old-table.

Warning! This option creates reverse triggers on the new table before it starts copying. After new table is renamed to its original name triggers will continue working. But because the name change metadata version in the table cache will also change you may start receiving “Prepared statement needs to be re-prepared” errors. The workaround for this is to re-prepare statements. If you do not use server-side prepared statements your application should not be affected.


type: DSN; repeatable: yes

DSN to skip when checking slave lag. It can be used multiple times. Example: –skip-check-slave-lag h=,P=12345 –skip-check-slave-lag h=,P=12346 Please take into consideration that even when for the MySQL driver h=127.1 is equal to h=, for this parameter you need to specify the full IP address.


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.


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.


type: Array

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

By default, the tool sets:


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.



Note the single backslash for the quotes and double backslash for the comma.


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.


short form: -S; type: string

Socket file to use for connection.


Print statistics about internal counters. This is useful to see how many warnings were suppressed compared to the number of INSERT.


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.


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):

===================  =====   ====
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:


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.


short form: -u; type: string

User for login if not current user.


Show version and exit.


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


default: yes

If replication is stopped, fail with an error (exit status 128) instead of waiting until replication is restarted.


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:


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') ) {
      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 on_copy_rows_after_nibble {
    my ($self, %args) = @_;
    print "PLUGIN on_copy_rows_after_nibble\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_die {
    my ($self, %args) = @_;
    print "PLUGIN before_die\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; };


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";


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.


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: no

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.


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.


Using <PTDEBUG> might expose passwords. When debug is enabled, all command line parameters are shown in the output.


INVALID_PARAMETERS              = 1
INVALID_PLUGIN_FILE             = 5
INVALID_KEY_SIZE                = 7
NOT_SAFE_TO_ASCEND              = 9
ERROR_ALTERING_TABLE            = 11
ERROR_UPDATING_FKS              = 15


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.


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.


Daniel Nichter and Baron Schwartz


The “online schema change” concept was first implemented by Shlomi Noach in his tool oak-online-alter-table, part of Engineers at Facebook then built another version called OnlineSchemaChange.php as explained by their blog post: This tool is a hybrid of both approaches, with additional features and functionality not present in either.


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 to learn about other free, open-source software from Percona.


pt-online-schema-change 3.5.7