Difference in Usage Mysql SELECT FOR UPDATE

Publish on Sep 13, 2024 by Alfin F

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I'd explain the differences between SELECT FOR UPDATE and SELECT FOR NO KEY UPDATE in MySQL:

SELECT FOR UPDATE:

Purpose: Acquires a lock on the selected rows, preventing other transactions from modifying or deleting them until the current transaction commits or rolls back.

Locking Mechanism: Uses a shared lock (S-lock) on each row, allowing other transactions to read the rows but preventing them from modifying or deleting them.

Usage Scenarios:
- Pessimistic locking: When you want to ensure data consistency and avoid conflicts during updates or deletions.
- Preventing race conditions: In scenarios where multiple transactions might try to update the same rows simultaneously.
- Implementing optimistic concurrency control: By combining SELECT FOR UPDATE with a subsequent UPDATE or DELETE statement, you can check for conflicts before making changes.

SELECT FOR NO KEY UPDATE:

Purpose: Acquires a lock on the selected rows, but only if they have a unique key or primary key. If a row doesn't have a unique key, no lock is acquired.

Locking Mechanism: Uses a shared lock (S-lock) on rows with unique keys, preventing other transactions from modifying or deleting them until the current transaction commits or rolls back.

Usage Scenarios:
- When you want to lock rows based on their uniqueness, but you don't need to lock rows that don't have a unique key.
- In situations where you want to avoid unnecessary locking overhead for rows that are unlikely to be modified or deleted.

Key Differences:

Locking Scope: SELECT FOR UPDATE locks all selected rows, while SELECT FOR NO KEY UPDATE only locks rows with unique keys.

Locking Granularity: SELECT FOR UPDATE locks individual rows, while SELECT FOR NO KEY UPDATE locks rows based on their uniqueness.

Performance Implications: SELECT FOR NO KEY UPDATE can be more efficient in scenarios where many rows don't have unique keys, as it avoids unnecessary locking overhead.

Choosing the Right Option:

Use SELECT FOR UPDATE when you need to ensure data consistency and prevent conflicts for all selected rows.

Use SELECT FOR NO KEY UPDATE when you want to lock rows based on their uniqueness but avoid unnecessary locking overhead for rows without unique keys.

Example:

-- SELECT FOR UPDATE: Locks all selected rows
SELECT * FROM my_table WHERE id > 10 FOR UPDATE;

-- SELECT FOR NO KEY UPDATE: Locks only rows with unique keys
SELECT * FROM my_table WHERE name = 'John' FOR NO KEY UPDATE;