wordpress/emlog中建表時key有什麼用？

<code>CREATE TABLE $wpdb->blogs (
  blog_id bigint(20) NOT NULL auto_increment,
  site_id bigint(20) NOT NULL default '0',
  domain varchar(200) NOT NULL default '',
  path varchar(100) NOT NULL default '',
  registered datetime NOT NULL default '0000-00-00 00:00:00',
  last_updated datetime NOT NULL default '0000-00-00 00:00:00',
  public tinyint(2) NOT NULL default '1',
  archived tinyint(2) NOT NULL default '0',
  mature tinyint(2) NOT NULL default '0',
  spam tinyint(2) NOT NULL default '0',
  deleted tinyint(2) NOT NULL default '0',
  lang_id int(11) NOT NULL default '0',
  PRIMARY KEY  (blog_id),
  KEY domain (domain(50),path(5)),
  KEY lang_id (lang_id)
) $charset_collate;

CREATE TABLE {$db_prefix}reply (
  id int(10) unsigned NOT NULL auto_increment,
  tid int(10) unsigned NOT NULL default '0',
  date bigint(20) NOT NULL,
  name varchar(20) NOT NULL default '',
  content text NOT NULL,
  hide enum('n','y') NOT NULL default 'n',
  ip varchar(128) NOT NULL default '',
  PRIMARY KEY  (id),
  KEY gid (tid),
  KEY hide (hide)
)</code>

尤其後一段程式碼裡，KEY gid (tid)是什麼意思？

回覆內容：

<code>CREATE TABLE $wpdb->blogs (
  blog_id bigint(20) NOT NULL auto_increment,
  site_id bigint(20) NOT NULL default '0',
  domain varchar(200) NOT NULL default '',
  path varchar(100) NOT NULL default '',
  registered datetime NOT NULL default '0000-00-00 00:00:00',
  last_updated datetime NOT NULL default '0000-00-00 00:00:00',
  public tinyint(2) NOT NULL default '1',
  archived tinyint(2) NOT NULL default '0',
  mature tinyint(2) NOT NULL default '0',
  spam tinyint(2) NOT NULL default '0',
  deleted tinyint(2) NOT NULL default '0',
  lang_id int(11) NOT NULL default '0',
  PRIMARY KEY  (blog_id),
  KEY domain (domain(50),path(5)),
  KEY lang_id (lang_id)
) $charset_collate;

CREATE TABLE {$db_prefix}reply (
  id int(10) unsigned NOT NULL auto_increment,
  tid int(10) unsigned NOT NULL default '0',
  date bigint(20) NOT NULL,
  name varchar(20) NOT NULL default '',
  content text NOT NULL,
  hide enum('n','y') NOT NULL default 'n',
  ip varchar(128) NOT NULL default '',
  PRIMARY KEY  (id),
  KEY gid (tid),
  KEY hide (hide)
)</code>

尤其後一段程式碼裡，KEY gid (tid)是什麼意思？

源自http://m.blog.csdn.net/article/details?id=51778816
key(sid,tid) 表示聯合主鍵，這兩個欄位的整體要唯一