前面说了system是存储引擎和只存一条数据，const是主键和索引才能达到的效率访问，ref是二级索引等值查询，或者联合索引全部等值，如果联合索引单个查询，则是index，ref_not是二级索引等值查询而且有null，range代表分区查询，all代表全表查询。
possible keys & key
possible keys代表可能用到的索引，key代表，mysql优化器成本计算后，实际会用到的索引key，
所以possible keys并不是越多越好，代表扫描的索引越多。
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z' AND key3 = 'a';
+----+-------------+-------+------------+------+-------------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+-------------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key1,idx_key3 | idx_key3 | 303 | const | 6 | 2.75 | Using where |
+----+-------------+-------+------------+------+-------------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
mysql> EXPLAIN SELECT key_part2 FROM s1 WHERE key_part3 = 'a';
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | s1 | NULL | index | NULL | idx_key_part | 909 | NULL | 9688 | 10.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+--------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)有一点特别的是，当type是index时候，possible_kyes是空的。
key_len
表示mysql当决定使用某个索引的时候，使用索引的大长度，这个长度是由三个部分组成：
1、对于使用固定长度类型的索引，则他实际占用的大小就是他的固定值，若是utf8，varchar(100)，则他实际占用长度是100*3 = 300。
2、如果有null，需要加个null值列表。长度为1
3、对于变长字段，都会有两个字节来存储实际长度。
mysql> EXPLAIN SELECT * FROM s1 WHERE id = 5;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key2 = 5;
+----+-------------+-------+------------+-------+---------------+----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | const | idx_key2 | idx_key2 | 5 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)从上可以看到，因为主键没有null值，所以比key2的key_len长度少1，因为key2可以有null。
对于可变长的索引来说：
mysql> EXPLAIN SELECT * 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 | NULL |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)因为key1属于varchar(100)，所以是100*3，又因为可以null，+1，又因为变长字段+2，所以就是303。当使用联合索引查询的时候，需要注意的是，会根据使用几个索引而现实多长：
mysql> EXPLAIN SELECT * FROM s1 WHERE key_part1 = 'a';
+----+-------------+-------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key_part | idx_key_part | 303 | const | 12 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 WHERE key_part1 = 'a' AND key_part2 = 'b';
+----+-------------+-------+------------+------+---------------+--------------+---------+-------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+--------------+---------+-------------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ref | idx_key_part | idx_key_part | 606 | const,const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+--------------+---------+-------------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)上面可以看到，联合索引里有三个字段，个sql用到一个索引，索引是303，第二个sql用到两个索引，索引是606.
ref
当索引在等值匹配查询的时候，在访问，const,ref，ref_or_null，unique sub_query，index sub_query其中之一时，
mysql> EXPLAIN SELECT * 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 | NULL |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)这里可以看到ref现实的是const，表名key1做匹配的时候，是个常数。
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s1.id = s2.id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------+
| 1 | SIMPLE | s1 | NULL | ALL | PRIMARY | NULL | NULL | NULL | 9688 | 100.00 | NULL |
| 1 | SIMPLE | s2 | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xiaohaizi.s1.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s2.key1 = UPPER(s1.key1);
+----+-------------+-------+------------+------+---------------+----------+---------+------+------+----------+-----------------------+
| 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 | func | 1 | 100.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+----------+---------+------+------+----------+-----------------------+
2 rows in set, 1 warning (0.00 sec)而这里可以看到前面的是数据库加s1的id，后面是一个函数。
row
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-----------------------+
| 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 (0.00 sec)如果是全表扫描，则是全表扫描的行数，如果是索引扫描，则是索引需要扫描的行数。比如上面的sql，因为key1是索引，所以是扫描索引满足的数量有266条。
Filtered
我们前面说过连接查询的时候，有一个扇出值的概念，被驱动表查询的次数，取决于驱动表查询的数据有多少行，
1、如果是全表扫描的时候，那么计算驱动表扇出时，估计出满足搜索记录需要多少条。
2、如果使用索引执行单表查询，那么计算驱动表扇出时，估计出满足使用到对应索引的搜索条件外其他搜索记录有多少条。
mysql> EXPLAIN SELECT * FROM s1 WHERE key1 > 'z' AND common_field = 'a';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+------------------------------------+
| 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 | 10.00 | Using index condition; Using where |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+------------------------------------+
1 row in set, 1 warning (0.00 sec)比方这个单表查询，rows可以看到key1索引扫描满足>z的有266条，那么有多少条满足common_filed = 'a' 呢，这就要看filtered了，sql查询出来的数据就是266 * 10%。
接下来我们 看连接查询
mysql> EXPLAIN SELECT * FROM s1 INNER JOIN s2 ON s1.key1 = s2.key1 WHERE s1.common_field = 'a';
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------+
| 1 | SIMPLE | s1 | NULL | ALL | idx_key1 | NULL | NULL | NULL | 9688 | 10.00 | Using where |
| 1 | SIMPLE | s2 | NULL | ref | idx_key1 | idx_key1 | 303 | xiaohaizi.s1.key1 | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+----------+---------+-------------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)连接查询可以看到s1表全表扫描查出估计值是9688条，然后* 10%，意味着扇出值就是9688 * 10%,也就意味着s2会执行扇出值的次数。
文章来源：知乎平台 原文地址：https://zhuanlan.zhihu.com/p/414667020
