SQL - Transactions and Data Integrity
Why do phantom reads occur even when a transaction reads the same query twice under REPEATABLE READ isolation?
Why do phantom reads occur even when a transaction reads the same query twice under REPEATABLE READ isolation?
hard📝 Conceptual Q10 of 15
Step-by-Step Solution
Solution:
Step 1: Understand REPEATABLE READ locking
REPEATABLE READ locks rows read but does not lock the range of rows scanned, so new rows can be inserted and appear on repeated queries.Step 2: Identify cause of phantom reads
Phantom reads happen because new rows appear in the range, which REPEATABLE READ does not prevent.Final Answer:
Because REPEATABLE READ does not lock range scans, allowing new rows to appear -> Option DQuick Check:
Phantom reads allowed by range scan gaps [OK]
Quick Trick: REPEATABLE READ misses range locks, causing phantom reads [OK]
Common Mistakes:
- Thinking REPEATABLE READ allows dirty reads
- Confusing phantom reads with non-repeatable reads
- Assuming lost updates cause phantom reads
Master "Transactions and Data Integrity" in SQL
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