CaseMySQL使用left join的时候or改写成union能提高效率
发布时间：2022-07-06 19:42:14 所属栏目：MySql教程 来源：互联网
导读：Case：MySQL使用left join的时候or改写成union可以提高效率 (1)优化前：使用or的时候，SQL执行时间1.47s mysql select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where e.emp_no=
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Case：MySQL使用left join的时候or改写成union可以提高效率 (1)优化前：使用or的时候，SQL执行时间1.47s mysql> select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where e.emp_no=32000 or d.from_date='1996-11-24'; 58 rows in set (1.47 sec) mysql> desc select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where e.emp_no=32000 or d.from_date='1996-11-24'; +----+-------------+-------+------------+------+----------------+---------+---------+--------------------+--------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+----------------+---------+---------+--------------------+--------+----------+-------------+ | 1 | SIMPLE | e | NULL | ALL | PRIMARY | NULL | NULL | NULL | 299335 | 100.00 | NULL | | 1 | SIMPLE | d | NULL | ref | PRIMARY,emp_no | PRIMARY | 4 | employees.e.emp_no | 1 | 100.00 | Using where | +----+-------------+-------+------------+------+----------------+---------+---------+--------------------+--------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec) mysql> select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where e.emp_no=32000 -> union -> select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where d.from_date='1996-11-24'; 58 rows in set (0.23 sec) mysql> desc select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where e.emp_no=32000 union select e.emp_no,e.first_name,d.dept_no,d.from_date,d.to_date from employees e left join dept_emp d on e.emp_no=d.emp_no where d.from_date='1996-11-24'; +----+--------------+------------+------------+--------+----------------+---------+---------+--------------------+--------+----------+-----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+------------+------------+--------+----------------+---------+---------+--------------------+--------+----------+-----------------+ | 1 | PRIMARY | e | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL | | 1 | PRIMARY | d | NULL | ref | PRIMARY,emp_no | PRIMARY | 4 | const | 1 | 100.00 | NULL | | 2 | UNION | d | NULL | ALL | PRIMARY,emp_no | NULL | NULL | NULL | 331143 | 10.00 | Using where | | 2 | UNION | e | NULL | eq_ref | PRIMARY | PRIMARY | 4 | employees.d.emp_no | 1 | 100.00 | NULL | | NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+------------+------------+--------+----------------+---------+---------+--------------------+--------+----------+-----------------+ 5 rows in set, 1 warning (0.00 sec)。 （编辑：航空爱好网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
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