mysql: Fix use of wrong SQL index in _getNextTID()

Similarly to 13911ca3 on the same instance after MariaDB was upgraded to 10.1.17 the following query, even after OPTIMIZE TABLE obj, started to execute very slowly:

MariaDB [(none)]> SELECT tid FROM neo1.obj WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
+--------------------+
| tid                |
+--------------------+
| 268707072758797063 |
+--------------------+
1 row in set (4.82 sec)

Both explain and analyze says the query will/is using partition key but only partially (note key_len is only 10, not 18):

MariaDB [(none)]> SHOW INDEX FROM neo1.obj;
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| obj   |          0 | PRIMARY   |            1 | partition   | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
| obj   |          0 | PRIMARY   |            2 | tid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
| obj   |          0 | PRIMARY   |            3 | oid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
| obj   |          0 | partition |            1 | partition   | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
| obj   |          0 | partition |            2 | oid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
| obj   |          0 | partition |            3 | tid         | A         |    28755928 |     NULL | NULL   |      | BTREE      |         |               |
| obj   |          1 | data_id   |            1 | data_id     | A         |    28755928 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
7 rows in set (0.00 sec)

MariaDB [(none)]> explain SELECT tid FROM neo1.obj WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
+------+-------------+-------+------+-------------------+-----------+---------+-------------+------+--------------------------+
| id   | select_type | table | type | possible_keys     | key       | key_len | ref         | rows | Extra                    |
+------+-------------+-------+------+-------------------+-----------+---------+-------------+------+--------------------------+
|    1 | SIMPLE      | obj   | ref  | PRIMARY,partition | partition | 10      | const,const |    2 | Using where; Using index |
+------+-------------+-------+------+-------------------+-----------+---------+-------------+------+--------------------------+
1 row in set (0.00 sec)

MariaDB [(none)]> analyze SELECT tid FROM neo1.obj WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
+------+-------------+-------+------+-------------------+-----------+---------+-------------+------+------------+----------+------------+--------------------------+
| id   | select_type | table | type | possible_keys     | key       | key_len | ref         | rows | r_rows     | filtered | r_filtered | Extra                    |
+------+-------------+-------+------+-------------------+-----------+---------+-------------+------+------------+----------+------------+--------------------------+
|    1 | SIMPLE      | obj   | ref  | PRIMARY,partition | partition | 10      | const,const |    2 | 9741121.00 |   100.00 |       0.00 | Using where; Using index |
+------+-------------+-------+------+-------------------+-----------+---------+-------------+------+------------+----------+------------+--------------------------+
1 row in set (4.93 sec)

By explicitly forcing (partition, oid, tid) index usage which is precisely designed to serve this and similar queries can avoid the query from being slow:

MariaDB [(none)]> analyze SELECT tid FROM neo1.obj FORCE INDEX(`partition`) WHERE `partition`=5 AND oid=79613 AND tid>268707071353462798 ORDER BY tid LIMIT 1;
+------+-------------+-------+-------+---------------+-----------+---------+------+------+--------+----------+------------+--------------------------+
| id   | select_type | table | type  | possible_keys | key       | key_len | ref  | rows | r_rows | filtered | r_filtered | Extra                    |
+------+-------------+-------+-------+---------------+-----------+---------+------+------+--------+----------+------------+--------------------------+
|    1 | SIMPLE      | obj   | range | partition     | partition | 18      | NULL |    2 |   1.00 |   100.00 |     100.00 | Using where; Using index |
+------+-------------+-------+-------+---------------+-----------+---------+------+------+--------+----------+------------+--------------------------+
1 row in set (0.00 sec)

/cc @jm, @vpelltier, @Tyagov