What are the transaction isolation levels in PostgreSQL and what anomalies do they protect against?

In PostgreSQL, there are four levels of transaction isolation defined by the SQL standard, which control the visibility of changes between transactions and protect against various anomalies:

1. Read Uncommitted

   • Allows reading even uncommitted changes of other transactions.
   • In PostgreSQL, it effectively behaves like Read Committed.
   • Anomalies: dirty reads are theoretically possible, but not in PostgreSQL.

2. Read Committed (default level)

   • A transaction sees only data that was committed at the start of each individual query.
   • Protects against dirty reads.
   • Non-repeatable reads and phantom reads are possible.

3. Repeatable Read

   • All queries within a transaction see the same snapshot of data taken at the beginning of the transaction.
   • Protects against dirty reads and non-repeatable reads.
   • Phantom reads are prevented in PostgreSQL thanks to MVCC.

4. Serializable

   • The strictest level.
   • Transactions execute as if they were run sequentially.
   • Protects against dirty reads, non-repeatable reads, and phantom reads.
   • Can lead to serialization errors if conflicts occur.

Thus, the choice of isolation level depends on the requirements for consistency and performance. Thanks to MVCC, even Read Committed in PostgreSQL provides protection against dirty reads, while Repeatable Read and Serializable offer stricter isolation.