SQL Server Isolation Levels: What's the Difference Between Read Committed and Repeatable Read?

SQL Server isolation level: the difference between read committed and repeatable read

SQL Server’s Read Committed and Repeatable Read isolation levels may look similar at first glance, but there are significant differences.

Read Committed

Read Committed ensures that any data read by the transaction is committed when it is read. This prevents transactions from seeing "dirty data", i.e. data that was in an intermediate state during an uncommitted transaction. However, read committed does not guarantee that subsequent reads of the same data will return the same value. Other transactions can still modify or delete data that has been read before the first transaction completes.

Example:

• Transaction A reads the row with value '1'.
• Transaction B updates the same row to '2' and commits.
• If transaction A re-reads the row before committing, it might see the old value ('1') or the new value ('2'), depending on the time.

Repeatable Read

Repeatable reads provide a higher level of isolation, ensuring:

• Data read during a transaction cannot be modified or deleted by other transactions before the first transaction is committed.
• As long as the first transaction remains uncommitted, subsequent reads of the same data will return the same value.

Example:

• Transaction A reads the row with value '1'.
• Transaction B attempts to update the same row, but is blocked until transaction A commits.
• If transaction A re-reads the row, it will always see the same value ('1') until it commits.

Summary

• Read committed prevents "dirty reads", but does not guarantee the consistency of subsequent reads.
• Repeatable reads ensure the consistency of subsequent reads and prevent other transactions from modifying the data that has been read before the first transaction commits.
• Higher isolation levels provide stronger data consistency guarantees, but may reduce concurrency and performance.

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