function db_filter_authors_search( $posts_search ) {

    // Don't modify the query at all if we're not on the search template or if the LIKE is empty
    if ( !is_search() || empty( $posts_search ) ) return $posts_search;

    global $wpdb;

    // Get all of the users of the blog and see if the search query matches either the display name or the user login
    add_filter( 'pre_user_query', 'db_filter_user_query' );

    $search = sanitize_text_field( get_query_var( 's' ) );

    $args = array(
        'count_total' => false,
        'search' => sprintf( '*%s*', $search ),
        'search_fields' => array(
            'display_name',
            'user_login',
        ),
        'fields' => 'ID',
    );

    $matching_users = get_users( $args );

    remove_filter( 'pre_user_query', 'db_filter_user_query' );

    // Don't modify the query if there aren't any matching users
    if ( empty( $matching_users ) ) return $posts_search;

    // Take a slightly different approach than core where we want all of the posts from these authors
    $posts_search = str_replace( ')))', ")) OR ( {$wpdb->posts}.post_author IN (" . implode( ',', array_map( 'absint', $matching_users ) ) . ")))", $posts_search );
    return $posts_search;
}
add_filter( 'posts_search', 'db_filter_authors_search' );

/**
 * Modify get_users() to search display_name instead of user_nicename. Helper function used inside of db_filter_authors_search()
 */
function db_filter_user_query( &$user_query ) {

    if ( is_object( $user_query ) )
        $user_query->query_where = str_replace( "user_nicename LIKE", "display_name LIKE", $user_query->query_where );
    return $user_query;
}