How Can I Perform a MySQL `ALTER TABLE` Operation Without Locking the Table and Disrupting Concurrent Updates?

ALTER TABLE Without Locking the Table

When performing an ALTER TABLE operation in MySQL, the table is read-locked for the duration of the statement, potentially blocking concurrent writes. If this operation is performed on a large table, INSERT or UPDATE statements could be disrupted for an extended period. Is there a solution to perform a "hot alter" without affecting ongoing updates?

The Only Alternative: Manual Manipulation

While MySQL lacks direct support for hot alters, an alternative exists: manually replicating the functionality that many RDBMS systems perform automatically. This involves:

1. Creating a New Table: Create an empty table with the desired column structure.
2. Copying Data: Transfer the data from the old table to the new table incrementally, while monitoring any concurrent operations on the original table.
3. Renaming Tables: Once the data migration is complete, rename the source table and the new table, ideally within a transaction.
4. Recompiling Objects: Update any stored procedures or other objects that reference the table, as the execution plans will need to be adjusted due to the structural changes.

Technical Considerations:

Adding a new field effectively modifies every row in the table. This process requires physical restructuring of the data on disk, similar to an extensive UPDATE operation with a greater impact. Field-level locking would be more complex than row-level locking, and table-wide locks are not always desirable.

Other RDBMS Solutions:

While MySQL may not support hot alters natively, other RDBMS systems may offer such functionality. For instance, PostgreSQL allows for partitioned tables, which can be altered individually without affecting other partitions.

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