How Does `SELECT ... FOR UPDATE` Ensure Data Consistency in Concurrent Database Access?

Concurrent Access with SELECT ... FOR UPDATE

Introduction

SELECT ... FOR UPDATE is a SQL statement used for locking database records during read operations. This ensures that the data remains unchanged while being accessed, preventing inconsistency issues.

Use Case for SELECT ... FOR UPDATE

Question 1:

The given scenario demonstrates a situation where SELECT ... FOR UPDATE can be beneficial. Thread 1 needs to list all rooms and their tags, but it's essential to know if a room has been removed. Using SELECT ... FOR UPDATE on rooms would prevent Thread 2 from deleting the room in question, ensuring Thread 1 retrieves accurate information despite the concurrent deletion operation.

Concurrent Isolation Levels

Question 2:

The choice between SERIALIZABLE and READ_COMMITTED isolation levels with SELECT ... FOR UPDATE depends on the database system being used.

MyISAM (MySQL): Tables are locked during queries, making SELECT ... FOR UPDATE unnecessary.

SQL Server: SELECT queries place shared locks on records, while DML queries place update locks. SELECT ... FOR UPDATE acts like an update lock, blocking concurrent delete operations.

MVCC (Oracle, PostgreSQL, MySQL with InnoDB): Read and write operations do not typically block each other. However, SELECT ... FOR UPDATE creates a special lock that prevents the deletion of locked records, similar to SQL Server's behavior.

REPEATABLE READ vs SERIALIZABLE

Question 2 (cont.):

• Oracle and PostgreSQL: In older versions, REPEATABLE READ is synonymous with SERIALIZABLE, meaning changes made after the transaction starts are not visible. SELECT ... FOR UPDATE may still be required to prevent phantom rows.
• MySQL InnoDB: SERIALIZABLE prevents concurrent DML on locked records, while REPEATABLE READ does not. Hence, SELECT ... FOR UPDATE is needed with REPEATABLE READ or READ COMMITED.

Conclusion

The use of SELECT ... FOR UPDATE in conjunction with appropriate isolation levels is crucial for maintaining data consistency during concurrent database access. However, the implementation details and required behaviors may vary depending on the underlying database system.

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