不会看 Explain执行计划，劝你简历别写熟悉 SQL优化
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2023-01-27 14:30:32
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小伙伴们有没有觉得学习数据库很有意思？有意思就对了！今天就给大家带来《不会看 Explain执行计划，劝你简历别写熟悉 SQL优化》，以下内容将会涉及到MySQL、Java、explain、简历，若是在学习中对其中部分知识点有疑问，或许看了本文就能帮到你！
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昨天中午在食堂，和部门的技术大牛们坐在一桌吃饭，作为一个卑微技术渣仔默默的吃着饭，听大佬们高谈阔论，研究各种高端技术，我TM也想说话可实在插不上嘴。
聊着聊着突然说到他上午面试了一个工作6年的程序员，表情挺复杂，他说：我看他简历写着熟悉
mysql> select version() from dual; +------------+ | version() | +------------+ | 5.7.17-log | +------------+
我们创建三张表
mysql> EXPLAIN SELECT * FROM one o,two t, three r WHERE o.two_id = t.two_id AND t.three_id = r.three_id; +----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+ | 1 | SIMPLE | o | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL | | 1 | SIMPLE | t | NULL | ALL | PRIMARY | NULL | NULL | NULL | 2 | 50 | Using where; Using join buffer (Block Nested Loop) | | 1 | SIMPLE | r | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.t.three_id | 1 | 100 | NULL | +----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
2、mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id from three r where r.three_name='我是第三表2'));
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | o | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 2 | SUBQUERY | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 3 | SUBQUERY | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
##### 3、以上两种同时存在
将上边的
mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id from three r where r.three_name='我是第三表2')) AND o.one_id in(select one_id from one where o.one_name="我是第一表2"); +----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+ | 1 | PRIMARY | o | NULL | ALL | PRIMARY | NULL | NULL | NULL | 2 | 50 | Using where | | 1 | PRIMARY | one | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.o.one_id | 1 | 100 | Using index | | 2 | SUBQUERY | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where | | 3 | SUBQUERY | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where | +----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
二、select_type
mysql> EXPLAIN select t.two_name, ( select one.one_id from one) o from (select two_id,two_name from two where two_name ='') t union (select r.three_name,r.three_id from three r); +------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+ | 1 | PRIMARY | two | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where | | 2 | SUBQUERY | one | NULL | index | NULL | PRIMARY | 4 | NULL | 2 | 100 | Using index | | 4 | UNION | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL | | NULL | UNION RESULT | <union1> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+</union1>
三、table
查询的表名，并不一定是真实存在的表，有别名显示别名，也可能为临时表，例如上边的
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | one | p201801,p201802,p201803,p300012 | index | NULL | PRIMARY | 9 | NULL | 3 | 100 | Using index | +----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
五、type
mysql> EXPLAIN SELECT * from three where three_id=1; +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | three | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100 | NULL | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
3、eq_ref
mysql> EXPLAIN select o.one_name from one o ,two t where o.one_id = t.two_id ; +----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+ | 1 | SIMPLE | o | NULL | index | PRIMARY | idx_name | 768 | NULL | 2 | 100 | Using index | | 1 | SIMPLE | t | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.o.one_id | 1 | 100 | Using index | +----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
4、ref
mysql> select o.one_id from one o where o.one_name = "xin" ; +--------+ | one_id | +--------+ | 1 | | 3 | +--------+
mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" ; +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | o | NULL | ref | idx_name | idx_name | 768 | const | 1 | 100 | Using index | +----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
5、ref_or_null
mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" OR o.one_name IS NULL; +----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+ | 1 | SIMPLE | o | NULL | ref_or_null | idx_name | idx_name | 768 | const | 3 | 100 | Using where; Using index | +----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
6、index_merge
mysql> EXPLAIN select * from one o where o.one_id >1 and o.one_name ='xin'; +----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+ | 1 | SIMPLE | o | NULL | index_merge | PRIMARY,idx_name | idx_name,PRIMARY | 772,4 | NULL | 1 | 100 | Using intersect(idx_name,PRIMARY); Using where | +----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
7、unique_subquery
value IN (SELECT primary_key FROM single_table WHERE some_expr)
8、index_subquery
value IN (SELECT key_column FROM single_table WHERE some_expr)
9、range
mysql> EXPLAIN SELECT * from three where three_id BETWEEN 2 AND 3; +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | three | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 1 | 100 | Using where | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
从结果中看到只有对设置了索引的字段，做范围检索
mysql> EXPLAIN SELECT * from three where user_id BETWEEN 2 AND 3; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | three | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
10、index
mysql> EXPLAIN SELECT three_id from three ; +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | three | NULL | index | NULL | PRIMARY | 4 | NULL | 1 | 100 | Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
11、ALL
mysql> EXPLAIN SELECT * from two ; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | two | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
六、possible_keys
mysql> EXPLAIN SELECT * from three; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | three | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
十一、filtered
mysql> EXPLAIN SELECT one_id from one ; +----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+ | 1 | SIMPLE | one | NULL | index | NULL | idx_two_id | 5 | NULL | 3 | 100 | Using index | +----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
注意：想要使用到覆盖索引，我们在
mysql> EXPLAIN SELECT * from one ; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | NULL | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
2、Using where
mysql> EXPLAIN SELECT one_name from one where create_time ='2020-05-18'; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
3、Using temporary
mysql> EXPLAIN SELECT one_name from one where one_id in (1,2) group by one_name; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | one | NULL | range| NULL | NULL | NULL | NULL | 3 | 33.33 | Using where; Using temporary; Using filesort | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
4、Using filesort
mysql> EXPLAIN SELECT one_id from one ORDER BY create_time; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | Using filesort | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
如果
mysql> EXPLAIN SELECT one_id from one ORDER BY one_id; +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | one | NULL | index | NULL | PRIMARY | 4 | NULL | 3 | 100 | Using index | +----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
5、Using join buffer
mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name; +----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+ | 1 | SIMPLE | o | NULL | index | idx_name | idx_name | 768 | NULL | 3 | 100 | Using where; Using index | | 1 | SIMPLE | t | NULL | ref | idx_name | idx_name | 768 | xin-slave.o.one_name | 1 | 100 | Using index | +----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
接下来删掉 连接条件
mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+ | 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL | | 1 | SIMPLE | o | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
6、Impossible where
mysql> EXPLAIN SELECT one_name from one WHERE 1=2; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
7、No tables used
mysql> EXPLAIN select now(); +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
Extra列的信息非常非常多，这里就不再一一列举了，详见
MySQL官方文档 ：https://dev.mysql.com/doc/ref...
总结
上边只是简单介绍了下
Explain执行计划各个列的含义，了解它不仅仅是要应付面试，在实际开发中也经常会用到。比如对慢
SQL进行分析，如果连执行计划结果都不会看，那还谈什么
SQL优化呢？
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