Read Committed vs. Repeatable Read in SQL Server: What's the Difference?

SQL Server's "Read Committed" and "Repeatable Read" isolation levels: in-depth difference analysis

Although SQL Server's "Read Committed" and "Repeatable Read" isolation levels both ensure data consistency, their key differences cannot be ignored.

Read Committed isolation level

Read Committed ensures that the data read by the transaction is committed before the transaction is visible. However, if data is updated by a concurrent transaction and the same data is read again by that transaction, the result consistency cannot be guaranteed.

Repeatable Read isolation level

In contrast, Repeatable Read provides a higher level of consistency. It not only guarantees that the data read is committed, but also that the data remains unchanged throughout the transaction. Even if other transactions update the data, the original read results remain consistent.

Example

Consider the following scenario, table T contains a column C with value '1'.

<code class="language-sql">BEGIN TRANSACTION;
SELECT * FROM T;
WAITFOR DELAY '00:01:00';
SELECT * FROM T;
COMMIT;</code>

• Read Committed: The second SELECT statement may return any updated or modified data, since the data may have changed during the delay.
• Repeatable Read: The second SELECT statement will always return the same rows originally read, ensuring data consistency.

Other isolation levels

In addition to Read Committed and Repeatable Read, SQL Server also provides other isolation levels:

• Serializable: Guarantees complete isolation, preventing any concurrent modification, deletion or insertion operations.
• Snapshot: Similar to Serializable, but uses snapshots to provide consistency without blocking concurrent transactions.

Choose the appropriate isolation level

The choice of isolation level depends on specific application requirements. Higher isolation levels, while ensuring consistency, may also impact scalability and performance. Read Committed is an appropriate isolation level for applications that can tolerate non-repeatable reads. Repeatable Read is recommended for applications that require consistency and cannot tolerate data changes during transaction execution.

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