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

The difference between Postgres and MySQL in this regard is that in Postgres it doesn't re-creates a table, but modifies data dictionary which is similar to Oracle. Therefore, the operation is fast, while it's still requires to allocate an exclusive DDL table lock for very short time as stated above by others.

In MySQL the operation will copy data to a new table while blocking transactions, which has been main pain for MySQL DBAs prior to v. 5.6.

The good news is that since MySQL 5.6 release the restriction has been mostly lifted and you now can enjoy the true power of the MYSQL DB.