What is a transaction and what are transaction isolation levels in PostgreSQL?

PostgreSQL supports four levels of transaction isolation, which determine how data modifications are visible in concurrent transactions:

1. Read Uncommitted — the lowest level of isolation. In PostgreSQL, it effectively behaves like Read Committed, meaning it does not allow seeing uncommitted changes from other transactions.

2. Read Committed (default level) — a transaction sees only the changes that were committed at the moment each query is executed. Different queries within the same transaction may see different data if other transactions have committed changes in between.

3. Repeatable Read — a transaction sees data as it was at the start of the transaction. All queries within the transaction see the same snapshot of data, even if other transactions modify and commit changes during that time.

4. Serializable — the strictest level of isolation. Transactions are executed as if they were run sequentially, one after another, preventing any anomalies due to concurrency. It may lead to transaction rollbacks in case of conflicts.

The choice of isolation level depends on data integrity requirements and performance considerations. For example, Read Committed offers good performance and is suitable for most cases, while Serializable is used when maximum consistency is needed.

A transaction is a sequence of operations on a database that is executed as a single unit. Either all operations of the transaction are successfully applied (commit), or none are applied (rollback), ensuring data integrity.

PostgreSQL supports the following transaction isolation levels, which determine how data modifications are visible to concurrent transactions:

1. Read Uncommitted — the lowest isolation level. Allows seeing uncommitted changes of other transactions (dirty read). In PostgreSQL, it effectively behaves like Read Committed.

2. Read Committed (default level) — a transaction sees only data that was committed at the start of each individual query. Changes made by other transactions after the start of the query are not visible.

3. Repeatable Read — a transaction sees data as it was at the start of the transaction. All queries within the transaction see the same snapshot of data, preventing non-repeatable reads, but phantom reads are possible.

4. Serializable — the strictest isolation level. Ensures that concurrent transaction execution is equivalent to some serial order. Prevents phantom reads and other anomalies but may lead to transaction rollbacks in case of conflicts.

Example of setting the isolation level in PostgreSQL:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- operations
COMMIT;