Can MySQL Perform ALTER TABLE Operations Without Locking the Entire Table?

ALTER TABLE without Table Lock: A Comprehensive Discussion

In MySQL, executing an ALTER TABLE statement incurs a read-lock on the entire table. While this allows for concurrent reads, write operations are blocked. For large tables, this can lead to significant delays in INSERT or UPDATE statements. To avoid this downtime, the question arises: is there a method to perform a "hot alter" that maintains table updatability during the process?

MySQL's Limitations

Unfortunately, MySQL does not offer a native solution for hot alters like adding a table column without locking the table. The traditional approach is to resort to a manual workaround.

Manual Solution

The alternative to a hot alter is a manual process:

1. Create a New Table: Create a new table with the desired added column.
2. Copy Data Chunk by Chunk: Gradually transfer data from the old table to the new one, while monitoring for any concurrent INSERT/UPDATE/DELETE operations on the source table.
3. Rename Tables: Change the names of the source and new tables to facilitate their usage in the production environment.
4. Recompile Procedures: Update any stored procedures or other database objects that reference the affected table.

Underlying Reasons for the Limitation

It is important to understand why MySQL does not support hot alters directly:

• Resemblance to Mass Update: Adding a column effectively updates all rows in the table with a new value.
• Physical Structure Modification: The alteration physically changes the storage structure of the table, requiring the movement of every record.

These factors make it difficult to implement field-level locking or perform hot alters without incurring some form of table lock.

Conclusion

While MySQL does not provide a built-in solution for hot alters, the manual workaround described above offers a viable alternative. It allows database administrators to avoid downtime during schema changes, ensuring uninterrupted access to the table throughout the process.

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