عرض عملي: انحراف الكتابة (write skew) تحت serializable

خطوة بخطوة

1. Rule: at least one admin must stay active. Table admin(id, active) with two active admins.
2. Under REPEATABLE READ, both transactions check "count of active > 1" and each deactivates a different admin; both commit and no admin remains.
3. Repeat with SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; one commit fails with 40001.
4. Planned mistake: run the application code without retry; the user sees an error. Fix: retry the whole transaction a limited number of times.
5. Mention the alternative: lock a parent row or use a constraint.

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