LaborC

I found the following code. Probably this can help to figure out a solution.

function my_smart_search( $search, &$wp_query ) {
    global $wpdb;

    if ( empty( $search ))
        return $search;

    $terms = $wp_query->query_vars[ 's' ];
    $exploded = explode( ' ', $terms );
    if( $exploded === FALSE || count( $exploded ) == 0 )
        $exploded = array( 0 => $terms );

    $search = '';
    foreach( $exploded as $tag ) {
        $search .= " AND (
            (wp_posts.post_title LIKE '%$tag%')
            OR (wp_posts.post_content LIKE '%$tag%')
            OR EXISTS
            (
                SELECT * FROM wp_comments
                WHERE comment_post_ID = wp_posts.ID
                    AND comment_content LIKE '%$tag%'
            )
            OR EXISTS
            (
                SELECT * FROM wp_terms
                INNER JOIN wp_term_taxonomy
                    ON wp_term_taxonomy.term_id = wp_terms.term_id
                INNER JOIN wp_term_relationships
                    ON wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id
                WHERE taxonomy = 'post_tag'
                    AND object_id = wp_posts.ID
                    AND wp_terms.name LIKE '%$tag%'
            )
        )";
    }

    return $search;
}

add_filter( 'posts_search', 'my_smart_search', 500, 2 );

from: http://vzurczak.wordpress.com/2013/06/15/extend-the-default-wordpress-search/