Explain MySQL's online DDL operations and their limitations.

Online DDL allows schema changes without blocking DML operations.

Algorithms:
• INSTANT: Metadata only changes
• INPLACE: No table copy required
• COPY: Full table rebuild

Example:
ALTER TABLE users ADD COLUMN phone VARCHAR(20), ALGORITHM=INPLACE;

Limitations:
• Some operations still require table copy
• Increased disk space during operation
• Performance impact on concurrent queries