MySQL Explain关键字：语法和输出格式

语法结构如下：

{EXPLAIN | DESCRIBE | DESC}tbl_name [col_name | wild]{EXPLAIN | DESCRIBE | DESC}[explain_type]{explainable_stmt | FOR CONNECTION connection_id}{EXPLAIN | DESCRIBE | DESC} ANALYZE [FORMAT = TREE] select_statementexplain_type: {FORMAT = format_name
}format_name: {TRADITIONAL| JSON| TREE
}explainable_stmt: {SELECT statement| TABLE statement| DELETE statement| INSERT statement| REPLACE statement| UPDATE statement
}

EXPLAIN | DESCRIBE | DESC 这三个关键字是同义词，通常使用DESCRIBE关键字获取表结构信息，而EXPLAIN用于获取查询执行计划。

EXPLAIN适用于 SELECT、 DELETE、 INSERT、 REPLACE和 UPDATE语句。在 MySQL 8.0.19 及更高版本中，它也适用于 TABLE语句。

当EXPLAIN与可解释语句一起使用时，MySQL 显示来自优化器的有关语句执行计划的信息。也就是说，MySQL 解释了它将如何处理该语句，包括有关表如何连接以及连接顺序的信息。

EXPLAIN 的输出格式

FORMAT选项可用于选择输出格式。默认的是TRADITIONAL,以表格格式呈现输出。也可以显式的设置为JSON格式显示信息。在 MySQL 8.0.16 及更高版本中，还可以设置为TREE提供树状输出，比 TRADITIONAL格式更精确地描述查询处理；使用EXPLAIN ANALYZE时，只能以TREE格式显示。

从 MySQL 8.0.32 开始， EXPLAIN默认输出格式由系统变量explain_format的值决定。

以下是使用TRADITIONAL时，输出为表格格式的内容。

mysql> EXPLAIN SELECT Name FROM country WHERE Code Like 'A%';
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | country | NULL       | range | PRIMARY       | PRIMARY | 12      | NULL |   17 |   100.00 | Using where |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

如果我们将FORMAT值设为 TREE，然后重新运行相同的EXPLAIN语句，输出将使用树状格式：

mysql> EXPLAIN FORMAT=TREE SELECT Name FROM country WHERE Code LIKE 'A%';
+--------------------------------------------------------------------------------------------------------------+
| EXPLAIN                                                                                                      |
+--------------------------------------------------------------------------------------------------------------+
| -> Filter: (country.`Code` like 'A%')  (cost=3.67 rows=17)-> Index range scan on country using PRIMARY over ('A' <= Code <= 'A????????')  (cost=3.67 rows=17)  |
+--------------------------------------------------------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

以下是JSON格式的输出

mysql> EXPLAIN FORMAT=JSON SELECT Name FROM country WHERE Code LIKE 'A%';
+------------------------------------------------------------------------------+
| EXPLAIN                                                                      |
+------------------------------------------------------------------------------+
| {"query_block": {"select_id": 1,"cost_info": {"query_cost": "3.67"},"table": {"table_name": "country","access_type": "range","possible_keys": ["PRIMARY"],"key": "PRIMARY","used_key_parts": ["Code"],"key_length": "12","rows_examined_per_scan": 17,"rows_produced_per_join": 17,"filtered": "100.00","cost_info": {"read_cost": "1.97","eval_cost": "1.70","prefix_cost": "3.67","data_read_per_join": "16K"},"used_columns": ["Code","Name"],"attached_condition": "(`world`.`country`.`Code` like 'A%')"}}
}                                                                              |
+------------------------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)