mysql explain的extra

時間 2020-06-16

標籤 mysql explain extra 欄目 MySQL 简体版

原文原文鏈接

導讀

extra主要有是那種狀況：Using index、Using filesort、Using temporary、Using wherejava

Using where無需多說，就是使用了where篩選條件。mysql

數據準備：sql

CREATE TABLE `t_blog` (
  `id` int(11) NOT NULL auto_increment,
  `title` varchar(50) default NULL,
  `typeId` int(11) default NULL,
  `a` int(11) default '0',
  PRIMARY KEY  (`id`),
  KEY `index_1` (`title`,`typeId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Using index

　　表示在查詢中使用了覆蓋索引，避免了掃描表的數據行。mysql優化

mysql> EXPLAIN select title from t_blog;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 | Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set

　　已知title字段是index_1索引的一部分，上條sql只查詢title字段，只會掃描索引文件而不會掃描表的全部數據行，在extra列中，出現了Using index。性能

mysql> EXPLAIN select * from t_blog;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 |       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set

　　上條語句中，除了查詢已經加了索引的字段，還查詢了沒有加索引的字段【a】，致使掃描了表的數據行，所以，extra列中沒有出現Using index。優化

　　當只出現Using index，沒出現Using where時，表示索引用於讀取數據，以第一條sql爲例。spa

　　當Using index 和 Using where同時出現時，表示索引用於查找動做，例如：code

mysql> EXPLAIN select title from t_blog where title = 'java';
+----+-------------+--------+------+---------------+---------+---------+-------+------+--------------------------+
| id | select_type | table  | type | possible_keys | key     | key_len | ref   | rows | Extra                    |
+----+-------------+--------+------+---------------+---------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | t_blog | ref  | index_1       | index_1 | 153     | const |    1 | Using where; Using index |
+----+-------------+--------+------+---------------+---------+---------+-------+------+--------------------------+
1 row in set

Using filesort

　　Using filesort一般出如今order by，當試圖對一個不是索引的字段進行排序時，mysql就會自動對該字段進行排序，這個過程就稱爲「文件排序」blog

mysql> EXPLAIN select * from t_blog order by title;
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+
|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 |       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+
1 row in set

　　已知title是index_1索引中的第一列索引，因此單獨使用時索引生效，在排序時根據索引排序，不會產生文件排序。排序

mysql> EXPLAIN select * from t_blog order by typeId;
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using filesort |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
1 row in set

　　雖然typeId是index_1索引的第二列，但因爲缺失第一列，因此索引失效。在排序時沒法根據索引排序，故mysql會自動進行排序，產生文件排序。

mysql> EXPLAIN select * from t_blog order by a;
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t_blog | ALL  | NULL          | NULL | NULL    | NULL |    7 | Using filesort |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
1 row in set

　　字段a上沒有任何索引，因此在排序時沒法根據索引排序，所以產生文件排序。

　　Using filesort出現的狀況：排序時沒法根據索引進行排序，mysql優化器只能本身進行排序，這種狀況會大大下降性能，不可取。

Using temporary

　　表示在查詢過程當中產生了臨時表用於保存中間結果。mysql在對查詢結果進行排序時會使用臨時表，常見於group by。

　　group by的實質是先排序後分組，同order by同樣，group by和索引息息相關。

　　試圖對一個沒有索引的字段進行分組，會產生臨時表：

mysql> EXPLAIN select title from t_blog group by typeId;
+----+-------------+--------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra                                        |
+----+-------------+--------+-------+---------------+---------+---------+------+------+----------------------------------------------+
|  1 | SIMPLE      | t_blog | index | NULL          | index_1 | 158     | NULL |    7 | Using index; Using temporary; Using filesort |
+----+-------------+--------+-------+---------------+---------+---------+------+------+----------------------------------------------+
1 row in set

　　對一個有索引的字段進行分組就不會產生臨時表：

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by b.id order by b.id;
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------+
|  1 | SIMPLE      | b     | index  | NULL          | PRIMARY | 4       | NULL          |    7 |       |
|  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |       |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-------+
2 rows in set

　　當order by子句和group by子句的字段不一樣時就會產生臨時表：

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by b.id order by b.title;
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-----------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra           |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-----------------+
|  1 | SIMPLE      | b     | index  | NULL          | index_1 | 158     | NULL          |    7 | Using temporary |
|  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |                 |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+-----------------+
2 rows in set

　　當時用left join時，若order by子句和group by子句都來自於從表時會產生臨時表：

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by t.id order by t.id;
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra                           |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+
|  1 | SIMPLE      | b     | ALL    | NULL          | NULL    | NULL    | NULL          |    7 | Using temporary; Using filesort |
|  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |                                 |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+
2 rows in set

mysql> explain select * from t_blog b left join t_type t on b.typeId = t.id group by t.id order by t.name;
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref           | rows | Extra                           |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+
|  1 | SIMPLE      | b     | ALL    | NULL          | NULL    | NULL    | NULL          |    7 | Using temporary; Using filesort |
|  1 | SIMPLE      | t     | eq_ref | PRIMARY       | PRIMARY | 4       | blog.b.typeId |    1 |                                 |
+----+-------------+-------+--------+---------------+---------+---------+---------------+------+---------------------------------+
2 rows in set

　　出現Using temporary意味着產生了臨時表存儲中間結果而且最後刪掉了該臨時表，這個過程很消耗性能。