The index owner needs the EXECUTE privilege on the
function used to define a function-based index. If the EXECUTE privilege is revoked,
Oracle marks the index DISABLED. The index owner does not need the EXECUTE
WITH GRANT OPTION privilege on this function to grant SELECT privileges on the
underlying table.

定义函数上的权限
1. 对于在函数索引中定义的函数 ， 用户必须具有EXECUTE权限
2. 如果EXECUTE权限被回收 ， 函数索引被标记为失效
3. 函数索引拥有者可赋予SELECT权限on underlying table 而不需要WITH GRANT OPTION.