UNION优化limit查询

mysql> explain
    -> (select first_name,last_name from sakila.actor order by last_name)
    -> union all
    -> (select first_name,last_name from sakila.customer order by last_name)
    -> limit 20;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | PRIMARY     | actor    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  200 |   100.00 | NULL  |
|  2 | UNION       | customer | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  599 |   100.00 | NULL  |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)


mysql> explain
    -> (select first_name,last_name from sakila.actor order by last_name limit 20)
    -> union all
    -> (select first_name,last_name from sakila.customer order by last_name limit 20)
    -> limit 20;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | PRIMARY     | actor    | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  200 |   100.00 | Using filesort |
|  2 | UNION       | customer | NULL       | ALL  | NULL          | NULL | NULL    | NULL |  599 |   100.00 | Using filesort |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+----------------+
2 rows in set, 1 warning (0.00 sec)

由上面的执行计划可知，两者的影响行数一样，但是加上union子句的limit 20 条限制，可以减少临时中间表中的数据（20+20，而不是 200+599）

可以从一定程度上优化查询。

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