急！关于基于函数的索引，我该如何写？

[email protected] >desc t
 名称                                                              是否为空? 类型
 ----------------------------------------------------------------- -------- ------------- V1                                                                         VARCHAR2(20)
 V2                                                                         VARCHAR2(20)[email protected] >create index ind_t on t(CONCAT(v1,v2));索引已创建。[email protected] >