Question

我的表格具有相同的架构

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当我执行此查询时，mysql使用了索引

CREATE TABLE `stock` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `currency` varchar(3) COLLATE utf8_unicode_ci NOT NULL,
  `against` varchar(3) COLLATE utf8_unicode_ci NOT NULL,
  `date` date NOT NULL,
  `time` time NOT NULL,
  `rate` double(8,4) NOT NULL,
  `ask` double(8,4) NOT NULL,
  `bid` double(8,4) NOT NULL,
  `created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  PRIMARY KEY (`id`),
  KEY `stock_currency_index` (`currency`),
  KEY `stock_against_index` (`against`),
  KEY `stock_date_index` (`date`),
  KEY `stock_time_index` (`time`),
  KEY `created_at_index` (`created_at`)
) ENGINE=InnoDB AUTO_INCREMENT=244221 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

此外，当我执行此查询时，mysql也使用了主索引

mysql> explain select max(id) from stock group by currency;
+----+-------------+-------+------------+-------+----------------------+----------------------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+----------------------+----------------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | stock | NULL       | range | stock_currency_index | stock_currency_index | 11      | NULL |    2 |   100.00 | Using index for group-by |
+----+-------------+-------+------------+-------+----------------------+----------------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

但当我结合这两个查询PRIMARY索引没有使用...我很困惑。我做错了什么？

mysql> explain select * from stock where id in (244221, 244222);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | stock | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

Answer 1

首先，尝试将查询重写为：

select s.*
from stock s join
     (select max(id) as maxid
      from stock
      group by currency
     ) ss
     on ss.maxid = s.id;

其次，我很想在stock(currency, id)上放一个索引并使用：

select s.*
from stock s
where s.id = (select max(s2.id) from stock s2 where s2.currency = s.currency);

这些中的任何一个都表现得更好吗？

MySQL与子查询结果进行比较时不使用主索引

1 个答案: