MySQL not using primary index when it had compare with subquery result - mysql

Question

I has table with the same schema
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;
When i execute this query mysql has using index
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)
Also when i am executing this query mysql has using primary index too
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)
BUT when i am combine these two queries PRIMARY index are not using... i am confused. What i am doing wrong?
mysql> explain select * from stock where id in (select max(id) from stock group by currency);
+----+-------------+-------+------------+-------+----------------------+----------------------+---------+------+--------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+----------------------+----------------------+---------+------+--------+----------+--------------------------+
| 1 | PRIMARY | stock | NULL | ALL | NULL | NULL | NULL | NULL | 221800 | 100.00 | Using where |
| 2 | SUBQUERY | stock | NULL | range | stock_currency_index | stock_currency_index | 11 | NULL | 2 | 100.00 | Using index for group-by |
+----+-------------+-------+------------+-------+----------------------+----------------------+---------+------+--------+----------+--------------------------+
2 rows in set, 1 warning (0.00 sec)

Answer 1

First, try rewriting the query as:
select s.*
from stock s join
(select max(id) as maxid
from stock
group by currency
) ss
on ss.maxid = s.id;
Second, I would be tempted to put an index on stock(currency, id) and to use:
select s.*
from stock s
where s.id = (select max(s2.id) from stock s2 where s2.currency = s.currency);
Do either of these perform better?