How to do an ALTER TABLE with a table in-use

Question

I have a script that runs 24-7 on a table to perform necessary functions on it. However, when it is running, it is almost impossible to do an ALTER TABLE ADD INDEX statement, as it seems like it just hangs indefinitely. Is there any way around this? How should I go about adding this index?

Answer 1

The Alter table statement is getting a metadata lockout. You cannot perform your alter statement while another transaction is in process on the same table. Since your script runs 24-7, it is not possible to do what you are asking.

To ensure transaction serializability, the server must not permit one session to perform a data definition language (DDL) statement on a table that is used in an uncompleted explicitly or implicitly started transaction in another session. The server achieves this by acquiring metadata locks on tables used within a transaction and deferring release of those locks until the transaction ends. A metadata lock on a table prevents changes to the table's structure. This locking approach has the implication that a table that is being used by a transaction within one session cannot be used in DDL statements by other sessions until the transaction ends.

You can read more about this Here at dev.mysql.

Answer 2

Version 5.6 has ALTER TABLE ... ALGORITHM=INLACE ... to do ADD INDEX and several other ALTERs without blocking everything.

pt-online-table-alter (from Percona.com) can do it in older versions of MySQL. It uses a TRIGGER.