2021年5月3日 (一) 11:13的版本

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

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

@@ 第24行： / 第24行： @@
 ;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