以下是phpbb的一个表的建表句，中间红色部分，包括一个primary key和其它的几个key,请问其它的这几个key这样设计是作什么用场的，请大侠们帮忙解豁，谢谢

#
# 数据表的结构 `phpbb_posts`
#

CREATE TABLE phpbb_posts (
post_id mediumint(8) unsigned NOT NULL auto_increment,
topic_id mediumint(8) unsigned NOT NULL default '0',
forum_id smallint(5) unsigned NOT NULL default '0',
poster_id mediumint(8) NOT NULL default '0',
post_time int(11) NOT NULL default '0',
poster_ip varchar(8) NOT NULL default '',
post_username varchar(25) default NULL,
enable_bbcode tinyint(1) NOT NULL default '1',
enable_html tinyint(1) NOT NULL default '0',
enable_smilies tinyint(1) NOT NULL default '1',
enable_sig tinyint(1) NOT NULL default '1',
post_edit_time int(11) default NULL,
post_edit_count smallint(5) unsigned NOT NULL default '0',
PRIMARY KEY (post_id),
KEY forum_id (forum_id),
KEY topic_id (topic_id),
KEY poster_id (poster_id),
KEY post_time (post_time)
) TYPE=MyISAM;