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Transaction isolation levels (e.g. Read Uncommitted, Read Committed, Repeatable Read, Serializable) in the official SQL standard are defined in terms of transaction anomalies like Read Skew, Lost Updates, etc. But the SQL standard itself is ambiguous (yes, even the latest 2023 version) and allows some silly behavior.
Let's motivate this entire article by looking at the Dirty Writes. If a transaction can overwrite the writes of another concurrent (uncommitted) transaction, we might have Dirty Writes. If a transaction can see the writes of another concurrent (uncommitted) transaction, we might have Dirty Reads. No major database allows Dirty Writes, even though the SQL specification allows it. But Dirty Reads are allowed by databases with Read Uncommitted isolation levels (PostgreSQL doesn't have such a level).
Lorin Hochstein has a thought experiment for Dirty Writes. Roughly put, in a bowling alley, no one should be able to acquire a half a pair of shoes within concurrent transactions. We'll adapt this thought experiment to a format we’ll be able to execute on multiple databases at multiple isolation levels with a new open-source tool, Monastery.
DROP TABLE IF EXISTS shoes;
CREATE TABLE shoes (left_shoe TEXT, right_shoe TEXT, shoe_id INT PRIMARY KEY);
INSERT INTO shoes VALUES ('', '', 1);
---
t1: BEGIN;
t2: BEGIN;
t1: UPDATE shoes SET left_shoe = 'Lin' WHERE shoe_id = 1;
t2: UPDATE shoes SET left_shoe = 'Carlos' WHERE shoe_id = 1;
t2: UPDATE shoes SET right_shoe = 'Carlos' WHERE shoe_id = 1;
t1: UPDATE shoes SET right_shoe = 'Lin' WHERE shoe_id = 1;
t1: SELECT * FROM shoes; -- # Inconsistent results here would mean dirty reads not necessarily dirty writes.
t2: SELECT * FROM shoes; -- # Inconsistent results here would mean dirty reads not necessarily dirty writes.
t1: COMMIT;
t2: COMMIT;
t1: SELECT * FROM shoes; -- assert ({Lin, Lin, 1}) or ({Carlos, Carlos, 1})
t2: SELECT * FROM shoes; -- assert ({Lin, Lin, 1}) or ({Carlos, Carlos, 1})
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About the author
Phil is the founder of The Consensus. Before this, he contributed to Postgres products at EnterpriseDB, cofounded and led marketing at TigerBeetle, and was an engineering manager at Oracle. He runs the Software Internals Discord, the Software Internals Email Book Club, and co-runs NYC Systems. @eatonphil
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