explain SQL语句性能检测

mysql>explain select * from table;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key? | key_len | ref? | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+

table????????? #显示该语句涉及的表
type?????????? #反映语句的质量，优-->差：const > eq_reg > ref > range > indexhe > ALL
possible_keys? #存在表中的所有索引
key??????????? #使用到的索引
key_len??????? #索引长度，越短越好
ref??????????? #显示索引的那一列被用到
rows?????????? #语句返回的数据行数
extra????????? #语句查询情况的详细信息