结贴了：字段排序规则不同，导致无法使用索引，我之前就发类似的问题。辛苦各位了，感谢感谢
+-----------------+----------------+--------------------+
| TABLE_NAME | COLUMN_NAME | COLLATION_NAME |
+-----------------+----------------+--------------------+
| user | mobile | utf8mb4_bin |
| user_risk | risk_value | utf8mb4_unicode_ci |
+-----------------+----------------+--------------------+