EXPLAIN output columns(解释输出列)
Each output row from EXPLAIN provides information about one table . Each cotains the values summarized in Table 8.1 "EXPLAIN Output Columns",and descirbed in more detail following the table .Column names are shown in the table's first cloumn;the second column provides the
equivalent property name shown in the output when FORMAT=JSON is used
EXPLAIN的每个输出行都提供有关一个表的信息。 每行都包含表8.1“ EXPLAIN输出列”中汇总的值，并在该表后进行了更详细的描述。 列名显示在表的第一列中； 第二列提供使用FORMAT = JSON时在输出中显示的等效属性名称。
Table 8.1 EXPLAIN Output columns
Columns JSON Name Meaning
id select_id the select identifiler
select_type none the select type
table table_name the table for the output row
partitions partitions the matching partitions
type access_type the join type
possible_keys possible_keys the possible indexes to choose
key key the index actually chosen
key_len key_length the length of the chosen key
ref ref the columns compared to the index
rows rows extimate of rows filtered by table condittion
filtered filtered percentage of rows filtered by table condtion
extra none additional information
MySQL官网的解释 type : the join type . 它描述了找到所需数据使用的扫描方式.