Can you explain what the transaction isolation level in PostgreSQL means and how it affects the system's operation?

The transaction isolation level in PostgreSQL determines how much a transaction can see the changes made by other transactions and affects the behavior of concurrent database operations.

Main isolation levels:

• Read Uncommitted: a transaction can see uncommitted changes of other transactions (in PostgreSQL, effectively equivalent to Read Committed).
• Read Committed (default level): a transaction only sees changes that were committed before the current read operation started.
• Repeatable Read: a transaction sees data as it was at the start of the transaction, even if other transactions modify data later.
• Serializable: the strictest level, providing full isolation as if transactions are executed sequentially.

Impact on the system:

• Higher isolation levels reduce the likelihood of concurrency issues (e.g., phantom reads) but may increase locking and decrease performance.
• Choosing an isolation level is a balance between data integrity and performance.