MySQL使用in帶子查詢的時候，子查詢不要使用union或union all
阿新 • • 發佈：2018-11-22
MySQL使用in帶子查詢的時候，子查詢不要使用union或union all
特別是當外部表比較大的時候，千萬不要使用in和union搭配，因為子查詢中一旦使用union，執行計劃會出現dependent subquery這種情況，
在生產上我們有使用類似的情況，導致SQL執行效率很差，下面舉例說明，為了生產安全隱私，以下舉例用測試表演示，原理相通。
舉例
使用in和union搭配的時候，s表作為外部表，全表掃描，有260w行，執行20多秒。
mysql> select s.* from salaries s where s.emp_no in (select emp_no from employees e where e.first_name='Georgi' union all select emp_no from employees e where e.hire_date='1992-12-18'); 2718 rows in set (21.14 sec) mysql> desc select s.* from salaries s where s.emp_no in (select emp_no from employees e where e.first_name='Georgi' union all select emp_no from employees e where e.hire_date='1992-12-18'); +----+--------------------+-------+------------+--------+---------------+---------+---------+------+---------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------------+-------+------------+--------+---------------+---------+---------+------+---------+----------+-------------+ | 1 | PRIMARY | s | NULL | ALL | NULL | NULL | NULL | NULL | 2612229 | 100.00 | Using where | | 2 | DEPENDENT SUBQUERY | e | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 10.00 | Using where | | 3 | DEPENDENT UNION | e | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 10.00 | Using where | +----+--------------------+-------+------------+--------+---------------+---------+---------+------+---------+----------+-------------+ 3 rows in set, 1 warning (0.00 sec)
可以使用join來轉化，再來看執行計劃e表變成外表，s表使用PK檢索，執行只要了0.32秒，效率大大提高。
mysql> select s.* from salaries s join (select emp_no from employees e where e.first_name='Georgi' union all select emp_no from employees e where e.hire_date='1992-12-18')e on s.emp_no=e.emp_no; 2718 rows in set (0.32 sec) mysql> desc select s.* from salaries s join (select emp_no from employees e where e.first_name='Georgi' union all select emp_no from employees e where e.hire_date='1992-12-18')e on s.emp_no=e.emp_no; +----+-------------+------------+------------+------+----------------+---------+---------+----------+--------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+------+----------------+---------+---------+----------+--------+----------+-------------+ | 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 59866 | 100.00 | NULL | | 1 | PRIMARY | s | NULL | ref | PRIMARY,emp_no | PRIMARY | 4 | e.emp_no | 9 | 100.00 | NULL | | 2 | DERIVED | e | NULL | ALL | NULL | NULL | NULL | NULL | 299335 | 10.00 | Using where | | 3 | UNION | e | NULL | ALL | NULL | NULL | NULL | NULL | 299335 | 10.00 | Using where | +----+-------------+------------+------------+------+----------------+---------+---------+----------+--------+----------+-------------+ 4 rows in set, 1 warning (0.00 sec)