TiDB 能保证自增值的单调性，但并不能保证其连续性。参考以下示例：
CREATE TABLE t (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, a VARCHAR(10), cnt INT NOT NULL DEFAULT 1, UNIQUE KEY (a));
INSERT INTO t (a) VALUES ('A'), ('B');
SELECT * FROM t;
INSERT INTO t (a) VALUES ('A'), ('C') ON DUPLICATE KEY UPDATE cnt = cnt + 1;
SELECT * FROM t;
+----+------+-----+
| id | a | cnt |
+----+------+-----+
| 1 | A | 1 |
| 2 | B | 1 |
+----+------+-----+
2 rows in set (0.00 sec)
Query OK, 3 rows affected (0.00 sec)
Records: 2 Duplicates: 1 Warnings: 0
+----+------+-----+
| id | a | cnt |
+----+------+-----+
| 1 | A | 2 |
| 2 | B | 1 |
| 4 | C | 1 |
+----+------+-----+
3 rows in set (0.00 sec)
https://docs.pingcap.com/zh/tidb/stable/auto-increment#单调性保证