Question

我正在处理一个很大的MySQL问题。看看这些查询。

CREATE TABLE `Users_Coupons_Views` (
 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
 `user_id` bigint(20) NOT NULL,
 `coupon_id` bigint(20) unsigned NOT NULL,
 `pos` int(10) unsigned DEFAULT NULL,
 `insert_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
 PRIMARY KEY (`id`),
 KEY `idx_cid` (`user_id`,`coupon_id`),
 KEY `idx_coupon_id` (`coupon_id`)
) ENGINE=MyISAM AUTO_INCREMENT=35423 DEFAULT CHARSET=utf8

CREATE TABLE `Track_Clicks_Processed` (
 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
 `track_id` bigint(20) unsigned NOT NULL,
 `user_id` bigint(20) unsigned NOT NULL,
 `ip` bigint(16) unsigned DEFAULT NULL,
 `coupon_id` bigint(20) unsigned NOT NULL DEFAULT '0',
 `merchant_id` int(11) NOT NULL DEFAULT '-1',
 `pos` int(11) NOT NULL DEFAULT '-1',
 `city` varchar(32) NOT NULL,
 `src` varchar(32) NOT NULL,
 `medium` varchar(32) NOT NULL,
 `kw` varchar(32) NOT NULL,
 `ref` varchar(255) NOT NULL,
 `user_agent` varchar(512) NOT NULL,
 `insert_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
 `ingr` tinyint(1) NOT NULL DEFAULT '0',
 `lang` char(2) NOT NULL DEFAULT 'it',
 PRIMARY KEY (`id`),
 KEY `idx_user_id` (`user_id`),
 KEY `idx_coupon_id` (`coupon_id`),
 KEY `idx_insert_time` (`insert_time`),
 KEY `idx_track_id` (`track_id`),
 KEY `idx_A1` (`track_id`),
 KEY `idx_cid` (`track_id`,`coupon_id`)
) ENGINE=MyISAM AUTO_INCREMENT=2997731 DEFAULT CHARSET=utf8

查询：

 EXPLAIN SELECT V.coupon_id, V.user_id, V.insert_time
FROM Track_Clicks_Processed AS C
JOIN Users_Coupons_Views AS V ON ( C.coupon_id = V.coupon_id
AND C.track_id = V.user_id )

结果如下：

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   SIMPLE  V   ALL     idx_cid,idx_coupon_id   NULL    NULL    NULL    17711   
1   SIMPLE  C   ref     idx_coupon_id,idx_track_id,idx_A1,idx_cid   idx_cid 16  yoodeal.V.user_id,yoodeal.V.coupon_id   2   Using where; Using index

它没有在第一个表上使用索引！为什么呢？

Answer 1

正如您在解释查询中所看到的，MySQL正在从第一个表中选择ALL行。这意味着索引无用，因为所有数据都已加载。

当我执行JOIN时，MySQL不使用索引

1 个答案: