WordPress posts_per_page如何将第一页的文章数设置成与其他页的数量不同

最近有个客户提到这样一个需要，就是首页先显示20篇文章，再每页加载10篇（点击加载更多），这样就与一般的分页不一样了，既要保证文章不重复，也要保证分页正确。请看下面代码：

function MBThemes_offset_firstpage( $query ) {
 if ( $query->is_main_query() && !is_admin() && $query->get('post_type') != 'project') {
 $offset = 10;
 $query->set( 'post_type', 'post' );
 $query->set( 'post_status', 'publish' );
 $query->set( 'ignore_sticky_posts', '-1' );
 $ppp = get_option('posts_per_page');
 
 if (!$query->is_paged()) {
 $query->set('posts_per_page',$offset + $ppp);
 } else {
 $offset = $offset + ( ($query->query_vars['paged']-1) * $ppp );
 $query->set('posts_per_page',$ppp);
 $query->set('offset',$offset);
 }
 }
}
add_action('pre_get_posts','MBThemes_offset_firstpage');

function MBThemes_offset_firstpage_pagination( $found_posts, $query ) {
 $offset = 10;

if( $query->is_main_query() && !is_admin() && $query->get('post_type') != 'project') {
 $found_posts = $found_posts - $offset;
 }
 return $found_posts;
}
add_filter( 'found_posts', 'MBThemes_offset_firstpage_pagination', 10, 2 );

来自：http://wordpress.stackexchange.com/questions/155758/have-different-number-of-posts-on-first-page