开发学院数据库MySQL MySQL中Join算法实现原理分析 阅读

MySQL中Join算法实现原理分析

 2009-04-04 11:18:13 来源:WEB开发网   
核心提示: 使用Query如下:1selectm.subjectmsg_subject,c.contentmsg_content23fromuser_groupg,group_messagem,group_message_contentc45whereg.user_id=167andm.group_i

使用Query如下:

1 select m.subject msg_subject, c.content msg_content
2 
3 from user_group g,group_message m,group_message_content c
4 
5 where g.user_id = 1
6 
7 and m.group_id = g.group_id
8 
9 and c.group_msg_id = m.id

 

看看我们的 Query 的执行计划:

1 sky@localhost : example 11:17:04> explain select m.subject msg_subject, c.content msg_content
2 
3 -> from user_group g,group_message m,group_message_content c
4 
5 -> where g.user_id = 1
6 
7 -> and m.group_id = g.group_id
8 
9 -> and c.group_msg_id = m.idG
10 
11 *************************** 1. row ***************************
12 
13 id: 1
14 
15 select_type: SIMPLE
16 
17 table: g
18 
19 type: ref
20 
21 possible_keys: user_group_gid_ind,user_group_uid_ind,user_group_gid_uid_ind
22 
23 key: user_group_uid_ind
24 
25 key_len: 4
26 
27 ref: const
28 
29 rows: 2
30 
31 Extra:
32 
33 *************************** 2. row ***************************
34 
35 id: 1
36 
37 select_type: SIMPLE
38 
39 table: m
40 
41 type: ref
42 
43 possible_keys: PRIMARY,idx_group_message_gid_uid
44 
45 key: idx_group_message_gid_uid
46 
47 key_len: 4
48 
49 ref: example.g.group_id
50 
51 rows: 3
52 
53 Extra:
54 
55 *************************** 3. row ***************************
56 
57 id: 1
58 
59 select_type: SIMPLE
60 
61 table: c
62 
63 type: ref
64 
65 possible_keys: idx_group_message_content_msg_id
66 
67 key: idx_group_message_content_msg_id
68 
69 key_len: 4
70 
71 ref: example.m.id
72 
73 rows: 2
74 
75 Extra:

 

Tags:MySQL Join 算法

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