mysql group by 超慢问题

mysql   group by 时超慢 ，group by 的字段还是 索引字段

看一下表结构

CREATE TABLE `cxy_act_code_verlog` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `dep_id` varchar(50) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `activity_id` varchar(50) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `openid` varchar(50) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `verify_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `ver_code` varchar(32) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL COMMENT '验证码',
  PRIMARY KEY (`id`),
  KEY `index_vlog_ver_openid` (`openid`) USING BTREE,
  KEY `index_ver_activity_id` (`activity_id`) USING BTREE,
  KEY `index_vlog_ver_code` (`ver_code`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=669378 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

看一下sql