表中没有足够的行让mysql的优化器来选择

1. mysql> CREATE TABLE `eos_softs` (
   
2.     ->   `id` int(10) NOT NULL auto_increment,
   
3.     ->   `name` varchar(50) NOT NULL default '',
   
4.     ->   `accessurl` varchar(50) default NULL,
   
5.     ->   `downloaded` int(20) default '0',
   
6.     ->   `picture` varchar(50) default NULL,
   
7.     ->   `source` varchar(25) default NULL,
   
8.     ->   `content` tinytext,
   
9.     ->   `type_id` int(5) default '0',
   
10.     ->   `url` tinyint(2) default '0',
    
11.     ->   `status` tinyint(2) default '1',
    
12.     ->   `keywords` varchar(50) default NULL,
    
13.     ->   `clicked` int(9) default '1',
    
14.     ->   `created` int(11) default NULL,
    
15.     ->   `modified` int(11) default NULL,
    
16.     ->   PRIMARY KEY  (`id`)
    
17.     -> ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=25 ;
    
18. Query OK, 0 rows affected (0.04 sec)
    
19. 
    
20. mysql> EXPLAIN SELECT *
    
21.     -> FROM eos_softs
    
22.     -> ORDER BY id DESC
    
23.     -> LIMIT 0 , 10 \G
    
24. *************************** 1. row ***************************
    
25.            id: 1
    
26.   select_type: SIMPLE
    
27.         table: eos_softs
    
28.          type: system
    
29. possible_keys: NULL
    
30.           key: NULL
    
31.       key_len: NULL
    
32.           ref: NULL
    
33.          rows: 0
    
34.         Extra: const row not found
    
35. 1 row in set (0.05 sec)
    
36. 
    
37. mysql> insert into eos_softs values();
    
38. Query OK, 1 row affected (0.00 sec)
    
39. 
    
40. mysql> EXPLAIN SELECT * FROM eos_softs ORDER BY id DESC LIMIT 0 , 10\G
    
41. *************************** 1. row ***************************
    
42.            id: 1
    
43.   select_type: SIMPLE
    
44.         table: eos_softs
    
45.          type: system
    
46. possible_keys: NULL
    
47.           key: NULL
    
48.       key_len: NULL
    
49.           ref: NULL
    
50.          rows: 1
    
51.         Extra:
    
52. 1 row in set (0.00 sec)
    
53. 
    
54. mysql> EXPLAIN SELECT id FROM eos_softs ORDER BY id DESC LIMIT 0 , 10\G
    
55. *************************** 1. row ***************************
    
56.            id: 1
    
57.   select_type: SIMPLE
    
58.         table: eos_softs
    
59.          type: system
    
60. possible_keys: NULL
    
61.           key: NULL
    
62.       key_len: NULL
    
63.           ref: NULL
    
64.          rows: 1
    
65.         Extra:
    
66. 1 row in set (0.00 sec)
    
67. 
    
68. mysql> insert into eos_softs values();                                 
    
69. Query OK, 1 row affected (0.00 sec)
    
70. 
    
71. mysql> insert into eos_softs values();
    
72. Query OK, 1 row affected (0.00 sec)
    
73. 
    
74. mysql> insert into eos_softs values();
    
75. Query OK, 1 row affected (0.00 sec)
    
76. 
    
77. mysql> insert into eos_softs values();
    
78. Query OK, 1 row affected (0.00 sec)
    
79. 
    
80. mysql> insert into eos_softs values();
    
81. Query OK, 1 row affected (0.00 sec)
    
82. 
    
83. mysql> EXPLAIN SELECT id FROM eos_softs ORDER BY id DESC LIMIT 0 , 10\G
    
84. *************************** 1. row ***************************
    
85.            id: 1
    
86.   select_type: SIMPLE
    
87.         table: eos_softs
    
88.          type: index
    
89. possible_keys: NULL
    
90.           key: PRIMARY
    
91.       key_len: 4
    
92.           ref: NULL
    
93.          rows: 6
    
94.         Extra: Using index
    
95. 1 row in set (0.00 sec)

复制代码