# isolation-levels

Published articles for isolation-levels.

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## Decoding isolation levels: I built a toy DB to force dirty reads and phantom reads

DevFeed: [Decoding isolation levels: I built a toy DB to force dirty reads and phantom reads](<https://devfeed.tech/articles/decoding-isolation-levels-i-built-a-toy-db-to-force-dirty-reads-and-phantom-reads-39587.md>)

Original publisher: [Read original article](<https://ankit-rana.com/logs/35-isolation-levels-toy-db-dirty-phantom-reads/>)

Author: hello@ankit-rana.com

Published: 2026-08-02T00:00:00Z

Content type: tutorial

Language: en

Sources: [Ankit Rana | Mechanical Sympathy](<https://devfeed.tech/sources/ankit-rana-mechanical-sympathy.md>)

Topics: [Databases](<https://devfeed.tech/topics/databases.md>), [PostgreSQL](<https://devfeed.tech/topics/postgresql.md>), [Python](<https://devfeed.tech/topics/python.md>), [MySQL](<https://devfeed.tech/topics/mysql.md>), [SQL](<https://devfeed.tech/topics/sql.md>), [locking](<https://devfeed.tech/topics/locking.md>)

Tags: [concurrency](<https://devfeed.tech/tags/concurrency.md>), [databases](<https://devfeed.tech/tags/databases.md>), [isolation-levels](<https://devfeed.tech/tags/isolation-levels.md>), [locking](<https://devfeed.tech/tags/locking.md>), [mvcc](<https://devfeed.tech/tags/mvcc.md>), [mysql](<https://devfeed.tech/tags/mysql.md>), [postgres](<https://devfeed.tech/tags/postgres.md>), [postgresql](<https://devfeed.tech/tags/postgresql.md>), [python](<https://devfeed.tech/tags/python.md>), [sql](<https://devfeed.tech/tags/sql.md>), [transactions](<https://devfeed.tech/tags/transactions.md>)

### AI overview

A hands-on tutorial uses a roughly 200-line Python toy database to reproduce transaction anomalies and explain why isolation levels behave differently across PostgreSQL and MySQL InnoDB. It shows that the ANSI SQL isolation table does not fully describe engine behavior, including PostgreSQL's handling of READ UNCOMMITTED and REPEATABLE READ, and MySQL InnoDB's snapshot and next-key locking behavior.

### Source excerpt

The ANSI isolation table lists which anomalies each level permits, but it does not describe what your engine actually does: Postgres silently upgrades READ UNCOMMITTED to READ COMMITTED, and its REPEATABLE READ prevents phantoms that the standard allows. Phantom reads are a locking problem rather than a row problem, because you cannot lock a row that does not exist yet. Snapshot isolation stops every anomaly the table names and still permits write skew, which the table never mentions.