Daniel Daniel - 1 month ago 14
MySQL Question

ALTER TABLE without locking the table?

When doing an ALTER TABLE statement in MySQL, the whole table is read-locked for the duration of the statement. If it's a big table, that means insert or update statements could be locked for a looooong time. Is there a way to do a "hot alter", like adding a column in such a way that the table is still updatable throughout the process?

Mostly I'm interested in a solution for MySQL but I'd be interested in other RDBMS if MySQL can't do it.

To clarify, my purpose is simply to avoid downtime when a new feature that requires an extra table column is pushed to production. Any database schema will change over time, that's just a fact of life. I don't see why we should accept that these changes must inevitably result in downtime; that's just weak.

Answer

The only other option is to do manually what many RDBMS systems do anyway...
- Create a new table

You can then copy the contents of the old table over a chunk at a time. Whilst always being cautious of any INSERT/UPDATE/DELETE on the source table. (Could be managed by a trigger. Although this would cause a slow down, it's not a lock...)

Once finished, change the name of the source table, then change the name of the new table. Preferably in a transaction.

Once finished, recompile any stored procedures, etc that use that table. The execution plans will likely no longer be valid.

EDIT:

Some comments have been made about this limitation being a bit poor. So I thought I'd put a new perspective on it to show why it's how it is...

  • Adding a new field is like changing one field on every row.
  • Field Locks would be much harder than Row locks, never mind table locks.

  • You're actually changing the physical structure on the disk, every record moves.
  • This really is like an UPDATE on the Whole table, but with more impact...