How to extend the structure of a heavily used database by adding a new column?

To add a new column to a heavily used database, you need to act carefully to avoid disrupting the application and causing downtime.

Main steps:

1. Planning changes — determine the type and properties of the new column, considering the impact on performance.
2. Adding a column with NULL or default value — this is a minimally blocking operation in most DBMSs.
3. Gradually updating data — if you need to fill the column with values, do it in batches to avoid overloading the database.
4. Updating application code — prepare to handle the new column, but initially maintain compatibility with missing data.
5. Testing on a test environment — verify that everything works correctly.

Example SQL for adding a column in MySQL:

ALTER TABLE users ADD COLUMN last_login DATETIME DEFAULT NULL;

If the database is very large and the operation locks the table, you can use online migration tools, such as pt-online-schema-change for MySQL, which allow changing the structure without long locks.