Explain isolation levels in SQL.

In the context of database transactions, isolation levels define how and when changes made by one transaction become visible to other concurrent transactions. They are a critical component of the ACID properties (Atomicity, Consistency, Isolation, Durability) and are crucial for maintaining data integrity and consistency in multi-user environments.

What are SQL Isolation Levels?

SQL isolation levels govern the degree to which a transaction must be isolated from the effects of other concurrent transactions. Higher isolation levels provide more data consistency but generally reduce concurrency, potentially impacting performance. Lower isolation levels increase concurrency but introduce the risk of various data anomalies.

Common Transaction Anomalies

To understand isolation levels, it's essential to know the common anomalies they aim to prevent:

• Dirty Read: A transaction reads data that has been written by another transaction but has not yet been committed. If the uncommitted transaction later rolls back, the first transaction will have read data that was never officially committed.
• Non-Repeatable Read: A transaction reads the same row twice and gets different values each time because another committed transaction modified that row between the two reads.
• Phantom Read: A transaction executes a query (e.g., SELECT * WHERE condition) and then re-executes the same query later, only to find a different set of rows (new rows inserted or existing rows deleted by another committed transaction that satisfy the WHERE condition).

ANSI SQL Standard Isolation Levels

The SQL standard defines four isolation levels, each preventing a specific set of these anomalies, building upon the previous one.

1. READ UNCOMMITTED

This is the lowest isolation level. It allows transactions to read uncommitted changes made by other transactions.

• Prevents: None of the common anomalies.
• Allows: Dirty reads, non-repeatable reads, phantom reads.
• Use Case: Rarely used, typically only in situations where approximate counts or highly tolerant statistics are acceptable, and maximum concurrency is paramount.

2. READ COMMITTED

This is a common default isolation level for many database systems (e.g., PostgreSQL, SQL Server). It ensures that a transaction only reads data that has been committed by other transactions.

• Prevents: Dirty reads.
• Allows: Non-repeatable reads, phantom reads.
• Use Case: Suitable for most general-purpose applications where reading the most recently committed data is acceptable, even if it changes upon re-reading.

3. REPEATABLE READ

This level ensures that if a transaction reads a row, any subsequent reads of that same row within the same transaction will yield the same value. It achieves this by preventing other transactions from modifying rows that have been read by the current transaction.

• Prevents: Dirty reads, non-repeatable reads.
• Allows: Phantom reads (new rows matching a query's criteria can still be inserted by other transactions according to the standard).
• Use Case: Applications where re-reading the same data within a transaction must consistently return the same values, such as reporting or aggregate calculations that span multiple queries.

4. SERIALIZABLE

This is the highest isolation level. It guarantees that concurrent transactions behave as if they were executed sequentially, one after the other. It effectively eliminates all concurrency anomalies.

• Prevents: Dirty reads, non-repeatable reads, phantom reads.
• Allows: None of the common anomalies.
• Use Case: Critical operations requiring absolute data consistency, where the overhead of reduced concurrency is acceptable. Often implemented using two-phase locking or optimistic concurrency control with serialization checks.

Summary of Anomalies Prevented

Setting Isolation Levels

Isolation levels can typically be set globally for a database session or specifically for individual transactions. The exact syntax may vary slightly between different database systems.

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

BEGIN TRANSACTION;
-- Your SQL statements here
COMMIT;