What isolation level is most commonly used with SELECT FOR UPDATE?

The most commonly used transaction isolation levels with SELECT FOR UPDATE are REPEATABLE READ or READ COMMITTED.

• REPEATABLE READ guarantees that data read in a transaction will not change until the transaction completes, which works well with row locking FOR UPDATE.
• READ COMMITTED is also frequently used, especially in high concurrency systems, as it only locks the selected rows for the duration of the transaction.

SELECT FOR UPDATE locks the selected rows to prevent concurrent modifications, and the isolation level should support these locks to avoid issues with unpredictable behavior.

Most DBMSs default to READ COMMITTED, which is a compromise between performance and consistency.