MySQL explain

Explain

Explain用来查看执行计划。可用于SELECT, DELETE, INSERT, REPLACE, and UPDATE statements

例子

mysql> explain SELECT COUNT(*) FROM vehicle_prod WHERE fin LIKE '%00000' \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: vehicle_prod
   partitions: NULL
         type: index
possible_keys: NULL
          key: idx_vehicle_prod_fin
      key_len: 203
          ref: NULL
         rows: 3182812
     filtered: 11.11
        Extra: Using where; Using index
1 row in set, 1 warning (0.00 sec)

其中：

id

The SELECT identifier

select_type

The SELECT type

• SIMPLE:Simple SELECT (not using UNION or subqueries)
• PRIMARY:Outermost SELECT
• UNION:Second or later SELECT statement in a UNION
• SUBQUERY:First SELECT in subquery
• ...

table

The table for the output row

partitions

The matching partitions

type

The join type

possible_keys

The possible indexes to choose

key

The index actually chosen

key_len

The length of the chosen key

ref

The columns compared to the index

rows

Estimate of rows to be examined

filtered

Percentage of rows filtered by table condition

Extra

Additional information

It is possible for key to name an index that is not present in the possible_keys value. This can happen if none of the possible_keys indexes are suitable for looking up rows, but all the columns selected by the query are columns of some other index. That is, the named index covers the selected columns, so although it is not used to determine which rows to retrieve, an index scan is more efficient than a data row scan.

Extra显示的索引扫描方式:

• using where：查询使用索引的情况下，需要回表去查询所需的数据。
• using index condition：查询使用了索引，但是需要回表查询数据。
• using index：查询使用覆盖索引的时候会出现。
• using index & using where：查询使用了索引，但是需要的数据都在索引列中能找到，不需要回表查询数据。

1. https://dev.mysql.com/doc/refman/8.0/en/explain-output.html