Databases · Middle · Question 62 of 75

Why can `ALTER TABLE` on a busy table freeze the whole application?

1It disconnects all clients
2ALTER TABLE always deletes the data
✓Correct answer: It waits for an exclusive lock; while waiting, it blocks every new query on that table behind it

Explanation

Set lock_timeout in migrations so the change fails fast instead of queuing everyone, and retry later.

Further reading

PostgreSQL: Explicit locking ↗

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