MySQL has no collation factor of order by

Before experiencing the problems encountered in production , In my memory, I think MySQL Of SELECT In none order by Will press ID Sort returns ~
But it's not ！！！

1. The following chapter is reproduced from ： https://segmentfault.com/a/1190000016251056

     I met one at work two days ago Mysql The question of sequencing , Without adding order by When , The order of acquired data is random , Instead of sorting by primary key . I used to mysql The default sorting of is by primary key . This is not the case .

①. Create a test database ：

CREATE TABLE `test` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `name` char(100) DEFAULT NULL,
  `age` char(5) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `age` (`age`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8

②. Insert some data ：

INSERT INTO test VALUES(NULL,' Zhang San ','5');
INSERT INTO test VALUES(NULL,' Li Si ','15');
INSERT INTO test VALUES(NULL,' Wang Wu ','5');
INSERT INTO test VALUES(NULL,' Zhao Xin ','15');
INSERT INTO test VALUES(NULL,' Dema ','20');
INSERT INTO test VALUES(NULL,' Prince ','5');
INSERT INTO test VALUES(NULL,' mywood ','17');
INSERT INTO test VALUES(NULL,' Hero ','22');
INSERT INTO test VALUES(NULL,' Water Margin ','18');
INSERT INTO test VALUES(NULL,' Xiaofang ','17');
INSERT INTO test VALUES(NULL,' Lao Wang ','5');

③. Inquire about 5 Bar record ：select * from test limit 5:

+----+------+------+
| id | name | age  |
+----+------+------+
|  1 |  Zhang San      | 5    |
|  2 |  Zhang San      | 5    |
|  3 |  Li Si      | 15   |
|  4 |  Wang Wu      | 5    |
|  5 |  Zhao Xin      | 15   |
+----+------+------+
5 rows in set (0.00 sec)

Now we only need to query two fields select id,age from test limit 5：

  +----+------+
| id | age  |
+----+------+
|  3 | 15   |
|  5 | 15   |
|  8 | 17   |
| 11 | 17   |
| 10 | 18   |
+----+------+
5 rows in set (0.00 sec)

Of two query statements Explain Implementation plan ：

mysql> explain select * from test limit 5 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: test
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 12
        Extra:
1 row in set (0.00 sec)

mysql> explain select id,age from test limit 5 \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: test
         type: index
possible_keys: NULL
          key: age
      key_len: 16
          ref: NULL
         rows: 12
        Extra: Using index
1 row in set (0.00 sec)

 
Conclusion ：
     MySQL Not given order by conditional , The order of the data results obtained is related to the query column . Because in different query columns , Different index conditions may be used .Mysql When using different indexes , The order of data obtained is different . This may be the same as Mysql Indexing mechanism , And the use of indexes . To avoid that , In future projects , Remember to add order by

 

2. seek truth

 
Refer to some personal and official articles , Roughly understand that in none order by Under the circumstances , influence MySQL The factors of sorting rules are ：

• SELECT Columns to query
• Index method
• physical position , That is, record the sequence of insertion rowid（ Not ID The order of ）
• Storage engine ,InnoDB、MyISM

[email protected] Of opinion ：
https://stackoverflow.com/questions/8746519/sql-what-is-the-default-order-by-of-queries

MySQL Official website answer ：