ALTER TABLE without locking the table?

Question

When doing an ALTER TABLE statement in MySQL, the whole table is read-locked (allowing concurrent reads, but prohibiting concurrent writes) for the duration of the statement

Answer 1

This question from 2009. Now MySQL offers a solution:

Online DDL (Data Definition Language)

A feature that improves the performance, concurrency, and availability of InnoDB tables during DDL (primarily ALTER TABLE) operations. See Section 14.11, “InnoDB and Online DDL” for details.

The details vary according to the type of operation. In some cases, the table can be modified concurrently while the ALTER TABLE is in progress. The operation might be able to be performed without doing a table copy, or using a specially optimized type of table copy. Space usage is controlled by the innodb_online_alter_log_max_size configuration option.

It lets you adjust the balance between performance and concurrency during the DDL operation, by choosing whether to block access to the table entirely (LOCK=EXCLUSIVE clause), allow queries but not DML (LOCK=SHARED clause), or allow full query and DML access to the table (LOCK=NONE clause). When you omit the LOCK clause or specify LOCK=DEFAULT, MySQL allows as much concurrency as possible depending on the type of operation.

Performing changes in-place where possible, rather than creating a new copy of the table, avoids temporary increases in disk space usage and I/O overhead associated with copying the table and reconstructing secondary indexes.

see MySQL 5.6 Reference Manual -> InnoDB and Online DDL for more info.

It seems that online DDL also available in MariaDB

Alternatively you can use ALTER ONLINE TABLE to ensure that your ALTER TABLE does not block concurrent operations (takes no locks). It is equivalent to LOCK=NONE.

MariaDB KB about ALTER TABLE