Class ReadLockMode (3.55.0)

    -  If isolation level is `REPEATABLE_READ`, then it is an
       error to specify `read_lock_mode`. Locking semantics
       default to `OPTIMISTIC`. No validation checks are done
       for reads, except for:

       1. reads done as part of queries that use
          `SELECT FOR UPDATE`
       2. reads done as part of statements with a
          `LOCK_SCANNED_RANGES` hint
       3. reads done as part of DML statements to validate that
          the data that was served at the snapshot time is
          unchanged at commit time.

    -  At all other isolation levels, if `read_lock_mode` is
       the default value, then pessimistic read lock is used.
PESSIMISTIC (1):
    Pessimistic lock mode.

    Read locks are acquired immediately on read. Semantics
    described only applies to `SERIALIZABLE` isolation.
OPTIMISTIC (2):
    Optimistic lock mode.

    Locks for reads within the transaction are not acquired on
    read. Instead the locks are acquired on a commit to validate
    that read/queried data has not changed since the transaction
    started. Semantics described only applies to
    `SERIALIZABLE` isolation.