What Are the Four Transaction Isolation Levels and What Problems Do They Solve?

Transaction Isolation Levels

Four Anomaly Types

• Dirty Read: Reading uncommitted data from another transaction
• Non-repeatable Read: Reading the same row twice in one transaction yields different results (another transaction updated it in between)
• Phantom Read: Two queries in one transaction return different row counts (another transaction inserted/deleted rows)
• Lost Update: Two concurrent updates to the same row; one overwrites the other

Four Isolation Levels (low to high)

Level	Dirty Read	Non-repeatable Read	Phantom Read
Read Uncommitted	Possible	Possible	Possible
Read Committed	Prevented	Possible	Possible
Repeatable Read	Prevented	Prevented	Possible
Serializable	Prevented	Prevented	Prevented

Performance Trade-off

Higher isolation = lower concurrency performance (more locking)

Default Levels

• MySQL (InnoDB): Repeatable Read
• PostgreSQL: Read Committed
• Oracle: Read Committed