What are the classifications of isolation levels?

Isolation levels in databases determine how isolated transactions are from each other and from changes made by other transactions. They prevent various issues of concurrent access.

Classification of isolation levels (SQL standard):

• READ UNCOMMITTED
  • Allows reading uncommitted changes (dirty read).
  • Problems: dirty read, non-repeatable read, phantom read.
• READ COMMITTED
  • Allows reading only committed changes.
  • Prevents dirty read.
  • Problems: non-repeatable read, phantom read.
  • The most commonly used default level.
• REPEATABLE READ
  • Ensures that re-reading the same set of rows within a transaction returns the same data, even if other transactions modify them.
  • Prevents dirty read and non-repeatable read.
  • Problem: phantom read (appearance of new rows matching the selection condition).
• SERIALIZABLE
  • Highest level of isolation.
  • Guarantees complete absence of concurrency issues. Transactions execute as if they were executed sequentially.
  • Prevents dirty read, non-repeatable read, and phantom read.
  • Can significantly impact performance.

Concurrency issues:

Issue	READ UNCOMMITTED	READ COMMITTED	REPEATABLE READ	SERIALIZABLE
Dirty read	Yes	No	No	No
Non-repeatable read	Yes	Yes	No	No
Phantom read	Yes	Yes	Yes	No

The choice of isolation level depends on application requirements and a trade-off between data integrity and performance.