MySQL5.7.11，tx_isolation为可重复读取;
像这样的table：

CREATE TABLE a (
  id int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

insert into a values(1);

在会话1中，执行如下命令：

begin;
select * from a where id=2 for update;

则在会话2中执行：

begin;
insert into a values(3);

session 2被阻塞，我认为session 2被阻塞是因为差距锁，但是在information_schema. inodb_lock中，它显示了上确界伪记录和RECORD锁;

*************************** 1. row ***************************
    lock_id: 234076:115:3:1
lock_trx_id: 234076
  lock_mode: X
  lock_type: RECORD
 lock_table: `test`.`a`
 lock_index: PRIMARY
 lock_space: 115
  lock_page: 3
   lock_rec: 1
  lock_data: supremum pseudo-record
*************************** 2. row ***************************
    lock_id: 234075:115:3:1
lock_trx_id: 234075
  lock_mode: X
  lock_type: RECORD
 lock_table: `test`.`a`
 lock_index: PRIMARY
 lock_space: 115
  lock_page: 3
   lock_rec: 1
  lock_data: supremum pseudo-record
2 rows in set (0.00 sec)

MySQL_Doc描述如下：

For the last interval, the next-key lock locks the gap above the largest value in the index and the
“supremum” pseudo-record having a value higher than any value actually in the index. The supremum
is not a real index record, so, in effect, this next-key lock locks only the gap following the largest index
value.

为什么记录锁和lock_data是上确界伪记录？