我有一张table，上面有大约50万条记录。

CREATE TABLE IF NOT EXISTS `p_transactions` (
  `transaction_id` bigint(10) unsigned NOT NULL,
  `amount` decimal(19,2) NOT NULL,
  `dt` bigint(1) NOT NULL,
  `transaction_status` int(1) NOT NULL,
  `transaction_type` varchar(15) NOT NULL,
  `payment_method` varchar(25) NOT NULL,
  `notes` text NOT NULL,
  `member_id` int(10) unsigned NOT NULL,
  `new_amount` decimal(19,2) NOT NULL,
  `paid_amount` decimal(19,2) NOT NULL,
  `secret_code` char(40) NOT NULL,
  `internal_status` varchar(40) NOT NULL,
  `ip_addr` varchar(15) NOT NULL,
  `description` text NOT NULL,
  `seller_transaction_id` varchar(50) DEFAULT NULL,
  `return_url` varchar(255) DEFAULT NULL,
  `fail_url` varchar(255) DEFAULT NULL,
  `success_url` varchar(255) DEFAULT NULL,
  `result_url` varchar(255) DEFAULT NULL,
  `user_fee` decimal(19,3) DEFAULT '0.000',
  `currency` char(255) DEFAULT 'USD',
  `gateway_transaction_id` char(255) DEFAULT NULL,
  `load_amount` decimal(19,2) NOT NULL,
  `transaction_mode` varchar(1) NOT NULL DEFAULT '',
  `p_fee` decimal(19,2) NOT NULL,
  `country` varchar(2) NOT NULL,
  `email` varchar(255) NOT NULL,
  `vat` decimal(19,2) NOT NULL DEFAULT '0.00',
  `name` varchar(255) NOT NULL,
  `bdate` varchar(255) NOT NULL,
  `child_method` varchar(255) NOT NULL,
  `processing_fee` decimal(19,2) NOT NULL DEFAULT '0.00',
  `flat_fee` varchar(1) NOT NULL DEFAULT 'n',
  `user_fee_sum` decimal(19,2) NOT NULL DEFAULT '0.00',
  `p_fee_sum` decimal(19,2) NOT NULL DEFAULT '0.00',
  `dt_open` bigint(1) NOT NULL DEFAULT '0',
  `user_fee_type` varchar(1) NOT NULL DEFAULT 'r',
  `custom_gateway_fee` decimal(19,2) NOT NULL DEFAULT '0.00',
  `paid_currency` varchar(3) NOT NULL DEFAULT 'USD',
  `paid_microtime` bigint(10) unsigned NOT NULL,
  `check_ballance` varchar(1) NOT NULL DEFAULT 'n',
  PRIMARY KEY (`transaction_id`),
  KEY `member_id` (`member_id`),
  KEY `payment_method` (`payment_method`),
  KEY `child_method` (`child_method`),
  KEY `check_ballance` (`check_ballance`),
  KEY `dt` (`dt`),
  KEY `transaction_type` (`transaction_type`),
  KEY `paid_microtime` (`paid_microtime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

当我执行查询时

SELECT *
FROM `p_transactions`
WHERE dt >= 1517443200
AND dt <= 1523404799
AND member_id =  2051
ORDER BY `paid_microtime` DESC
 LIMIT 50;

它运行0000秒。（+0016秒。网络）
但如果我加上这个条件 AND transaction_status = 7 ```
SELECT *
FROM p_transactions

WHERE dt >= 1517443200
AND dt <= 1523404799
AND member_id = 2051
AND transaction_status = 7

ORDER BY paid_microtime DESC
LIMIT 50

查询运行12938秒。（+0062秒。网络）
请帮我找出这种行为的原因
另外，上面有索引 `transaction_status` 而且它更增加了执行时间。