mysql - SQL - 嵌套查询的计数仍然很快吗？

Question

假设我有如下查询:

SELECT message.mid
FROM message
WHERE message.mid <= 100

据我所知，如果将查询更改为以下内容，它将执行得更快，因为列没有展开。

SELECT COUNT(message.mid)
FROM message
WHERE message.mid <= 100

但是下面的查询会有同样的好处吗？它还会那么快吗？

SELECT COUNT(*)
FROM (
    SELECT message.mid,
           message.something,
           message.something2,
           message.something3,
    FROM message
    WHERE message.mid <= 100
) AS A

Answer 1

We can ask MySQL what it will do .这是 5.7。

mysql> explain SELECT COUNT(*) FROM (     SELECT message.mid     FROM message     WHERE message.mid <= 100 
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | message | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |  100 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)


mysql> explain SELECT count(message.mid)     FROM message     WHERE message.mid <= 100;
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | message | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |  100 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

看起来是一样的。 MySQL 已经优化掉了子查询。

这是当 MySQL 未优化子查询时我们会看到的示例。

mysql> explain SELECT * FROM (     SELECT message.mid     FROM message where mid < 100 group by mid) m;
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | PRIMARY     | <derived2> | NULL       | ALL   | NULL          | NULL    | NULL    | NULL |   99 |   100.00 | NULL                     |
|  2 | DERIVED     | message    | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |   99 |   100.00 | Using where; Using index |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+

---

"Optimizing Derived Tables and View References"有此优化如何工作的示例。

Example 1:

SELECT * FROM (SELECT * FROM t1) AS derived_t1;

With merging, that query is executed similar to:

SELECT * FROM t1;

该页面概述了 MySQL 用来提高子查询效率的许多其他优化技巧。