If AUTOCOMMIT=1, then we do not need to make a plain SELECT set shared locks even on the SERIALIZABLE isolation level, because we know the transaction is read-only: a read-only transaction can always be performed on the REPEATABLE READ level, and that does not endanger the serializability


sql/ha_innodb.cc:
  If AUTOCOMMIT=1, then we do not need to make a plain SELECT set shared locks even on the SERIALIZABLE isolation level, because we know the transaction is read-only: a read-only transaction can always be performed on the REPEATABLE READ level, and that does not endanger the serializability