عرض عملي: تحديث مفقود في طرفيتين

خطوة بخطوة

1. Create stock(product_id, qty) with one row, qty 10.
2. In two psql terminals, both BEGIN; SELECT qty (10), then both UPDATE stock SET qty = 9 and commit; final qty is 9, not 8.
3. Fix 1: atomic UPDATE stock SET qty = qty - 1 WHERE qty > 0 (final 8).
4. Planned mistake: assume REPEATABLE READ always lets both commit; show T2 fails with a serialization error ("could not serialize access due to concurrent update"). Explain this is correct and needs a retry.
5. Fix 2: SELECT ... FOR UPDATE so the second waits.

Prepares MID-015-LAB1.

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