前面说了有type,simple表示普通查询或者连接查询，primary代表union左边的select，union result代表union查询的临时表去重，所以union all没有去重功能，subquery代表in的子查询物化表的情况下才会出现，dependent subquery代表相关子查询，dependent union代表相关union查询，还有driverd子查询，from后面的，也需要物化，还有物化后转连接查询，这些都能看到mysql优化器是采用哪种查询方式。
Id代表select，几个select就有几个查询，如果转链接了，就只有一个id。
Type代表存储引擎查询的方式，system代表查询并且只有一条记录，const，索引或者主键，ref，二级索引查询，ref_or_null，range，index代表联合索引但没有触发，range和ref，后还要all。
Possible key代表可能用到的索引。
Key实际用到的索引
Ref代表后面的过滤条件可能用到的方式，他可能是个函数，也可能是驱动表的参数。
Rows代表查询的数据，全表代表所有，索引则代表所有满足的数据。
Filtered代表数据多少满足，和rows组合可以算出去驱动表的扇出值。
Extra
顾名思义，这列就存储的是额外信息，我们可以通过额外信息准确理解mysql到底执行查询语句。
mysql> EXPLAIN SELECT 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
//当我们的sql语句没有表时候，extra显示的是no tables used。
mysql> EXPLAIN SELECT * FROM s1 WHERE 1 != 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
1 row in set, 1 warning (0.01 sec)
//当过滤条件后面永远显示false，就会出现Impossible where
mysql> EXPLAIN SELECT MIN(key1) FROM s1 WHERE key1 = 'abcdefg';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------------------+
| 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 matching min/max row |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------------------+
1 row in set, 1 warning (0.00 sec)
//当查询列表有max或者min，但没有查到数据时候，会显示：no matching min/max row
mysql> EXPLAIN SELECT key1 FROM s1 WHERE key1 = 'a';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 8 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
//当我们在使用索引覆盖的情况下，当我们只需要用到索引，而不需要回表操作，则显示 using index。
SELECT * FROM s1 WHERE key1 > 'z' AND key1 LIKE '%a';
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z' AND key1 LIKE '%b';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | s1 | NULL | range | idx_key1 | idx_key1 | 303 | NULL | 266 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.01 sec)
//有些虽然用了索引列，但不能使用到索引
//其中key1>'z'可以使用到索引，但后面的like不可以使用索引，
//传统访问应该先索引在索引b+树查询满足z的条件，然后数据回表查询过滤满足like '%a'的数据。
//但其实可以在索引b+树查询满足z的条件后，继续在b+树过滤索引满足key1 like '%a'的索引，因为key1
//也在索引b+树，这样就减少回表的开销。而且因为回表是随机I/O,这边是顺序I/O，效率也会快很多。
//mysql吧这种就称呼 索引条件下推，using index condition
mysql> EXPLAIN SELECT * FROM s1 WHERE common_field = 'a';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
//当我们全表扫描的时候，显示的是using where
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 = 'a' AND common_field = 'a';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | const | 8 | 10.00 | Using where |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
//当我们使用索引访问，但还有其他搜索条件还有common_Filed所以还是显示suing where
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s1.common_field = s2.common_field;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9954 | 10.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.03 sec)
//前面说过基于块的嵌套法，当被驱动表不能通过索引访问，这时候会有一个join buffer，来加快查询的速度，因为这个太多的话
//内存里不够存放足够多的数据，只能不断通过驱动表查询的结果，一条条从磁盘访问驱动表，
//这时候就把被驱动表的数据放在join buffer里，直接在内存中处理是否满足，就不需要每次都从磁盘查询数据。
//所以上面可以看到，因为不能使用索引扫描，所以退而求其次，用join buffer，
//又因为s2.common_field = 常数，所以这里又有一个using where
mysql> EXPLAIN SELECT * FROM s1 LEFT JOIN s2 ON s1.key1 = s2.key1 WHERE s2.id IS NULL;
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | ref | idx_key1 | idx_key1 | 303 | xiaohaizi.s1.key1 | 1 | 10.00 | Using where; Not exists |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------------------+
2 rows in set, 1 warning (0.00 sec)
//当我们在外连接，如果where条件包含被驱动表某个列等于null，但当前列有不允许为null，这时候就会显示not exists
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 = 'a' AND key3 = 'a';
+----+-------------+-------+------------+-------------+-------------------+-------------------+---------+------+------+----------+-------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+-------------------+-------------------+---------+------+------+----------+-------------------------------------------------+
| 1 | SIMPLE | s1 | NULL | index_merge | idx_key1,idx_key3 | idx_key3,idx_key1 | 303,303 | NULL | 1 | 100.00 | Using intersect(idx_key3,idx_key1); Using where |
+----+-------------+-------+------------+-------------+-------------------+-------------------+---------+------+------+----------+-------------------------------------------------+
1 row in set, 1 warning (0.01 sec)
//前面说过如果单独的索引，会使用合并索引，上面就显示用的using_intersect(idx_key3,idx_key1)
//如果是union则就用的是using_union合并
mysql> EXPLAIN SELECT * FROM s1 LIMIT 0;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------+
| 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 | Zero limit |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------+
1 row in set, 1 warning (0.00 sec)
//如果分页是0，则直接会显示zero limit
mysql> EXPLAIN SELECT * FROM s1 ORDER BY key1 LIMIT 10;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | index | NULL | idx_key1 | 303 | NULL | 10 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.03 sec
mysql> EXPLAIN SELECT * FROM s1 ORDER BY common_field LIMIT 10;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
//前面那个显示的null，对索引进行排序了
//但有些时候无法使用索引排序，这时候就需要从磁盘排序或者内存排序，这些都叫file sort
//如果用文件排序则就会显示using filesort
mysql> EXPLAIN SELECT DISTINCT common_field FROM s1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)
//有的时候mysql会使用临时表进行去重或者排序，比如我们在distinct，group by，union等子句的查询过程，
//这时候如果不能利用索引查询，这时候会建立内部临时表，这时候就会显示 Using temporary
mysql> EXPLAIN SELECT common_field, COUNT(*) AS amount FROM s1 GROUP BY common_field;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 100.00 | Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+---------------------------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT common_field, COUNT(*) AS amount FROM s1 GROUP BY common_field ORDER BY NULL;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | s1 | NULL | ALL | NULL | NULL | NULL | NULL | 9688 | 100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)
EXPLAIN SELECT common_field, COUNT(*) AS amount FROM s1 GROUP BY common_field ORDER BY common_field;
//上面的显示不光有using temporary 还有using filesort，也就是说不光临时表，还排序了。
//这是因为mysql默认在group by之后默认会order by,不想排序必须显示写 order by null
mysql> EXPLAIN SELECT key1, COUNT(*) AS amount FROM s1 GROUP BY key1;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | index | idx_key1 | idx_key1 | 303 | NULL | 9688 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
//另外建立临时表代价很大，能用索引 就用索引，如果用到索引group by 就会显示using index，并且索引是排序好的。
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 IN (SELECT key3 FROM s2 WHERE common_field = 'a');
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+------------------------------+
| 1 | SIMPLE | s2 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9954 | 10.00 | Using where; Start temporary |
| 1 | SIMPLE | s1 | NULL | ref | idx_key1 | idx_key1 | 303 | xiaohaizi.s2.key3 | 1 | 100.00 | End temporary |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+------------------------------+
2 rows in set, 1 warning (0.00 sec)
//当我们查询优化器吧子查询in转为semi-join的时候，有很多执行策略，其实一种也是建立临时表实现为外层查询进去重操作
//这时候驱动表start temporary 被驱动表将会显示end temporary
mysql> EXPLAIN SELECT * FROM s1 WHERE key3 IN (SELECT key1 FROM s2 WHERE key1 > 'z');
+----+-------------+-------+------------+-------+---------------+----------+---------+-------------------+------+----------+-------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+-------------------+------+----------+-------------------------------------+
| 1 | SIMPLE | s2 | NULL | range | idx_key1 | idx_key1 | 303 | NULL | 270 | 100.00 | Using where; Using index; LooseScan |
| 1 | SIMPLE | s1 | NULL | ref | idx_key3 | idx_key3 | 303 | xiaohaizi.s2.key1 | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+----------+---------+-------------------+------+----------+-------------------------------------+
2 rows in set, 1 warning (0.01 sec)
//内连接还会显示looseScan
mysql> EXPLAIN SELECT * FROM s1 WHERE common_field IN (SELECT key1 FROM s2 where s1.key3 = s2.key3);
+----+-------------+-------+------------+------+-------------------+----------+---------+-------------------+------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+-------------------+----------+---------+-------------------+------+----------+-----------------------------+
| 1 | SIMPLE | s1 | NULL | ALL | idx_key3 | NULL | NULL | NULL | 9688 | 100.00 | Using where |
| 1 | SIMPLE | s2 | NULL | ref | idx_key1,idx_key3 | idx_key3 | 303 | xiaohaizi.s1.key3 | 1 | 4.87 | Using where; FirstMatch(s1) |
+----+-------------+-------+------------+------+-------------------+----------+---------+-------------------+------+----------+-----------------------------+
2 rows in set, 2 warnings (0.00 sec)
//内连接还会显示firstMatch文章来源：知乎平台 原文地址：https://zhuanlan.zhihu.com/p/415060489
