如何修复大型数据库中的“超时”错误，从连接表中查找第二个修改后的结果？

I have 3 MySQL Tables user, board and pin. a user tables have around 66 columns. A pin table have 89 columns and A board table have 12 columns. A user table have almost 35000 users, pin have .3 million records and boards have 7k boards. A user can have multi boards and A board can contain multi-pins. I need last modified piece of each user so i could send them email if there is no update from a very long period of time.

I have tried the following code for a moment in codeigniter.

$this->db->select("t1.id as pin_id, t1.id, t1.user_id, `t1.title`,t1.image, t1.description, t1.profession_tag, t1.industry_tag, t1.collaborators, `t2.title` as board_title, t3.email, t3.username, t3.firstname");
        $this->db->from("pin as t1");
        $this->db->join('board as t2', 't1.board_id = t2.id', 'left');
        $this->db->join('user as t3', 't1.user_id = t3.id', 'left');
        $this->db->where('t3.status', 1);
        $this->db->where('t3.hidden_profile', 1);
        $this->db->where('t1.status', 1);
        $this->db->group_by('t1.user_id');
        $this->db->limit($limit, $start);
        $query = $this->db->get();
        return $query->result_array();

But in cases a user have more than 1000 pins its taking so much time. Please suggest what rest i can do to improve it and get the result faster. I am also not getting the last modified result yet by the query. I don't know how it will work with Having clause.

One suggestion is below. Use Sub queries in the Join statement. See this answer by Trendfischer and An Essential Guide to MySQL Derived Table

$this->db->select("t1.id as pin_id, t1.id, t1.user_id, `t1.title`,t1.image, t1.description, t1.profession_tag, t1.industry_tag, t1.collaborators, `t2.title` as board_title, t3.email, t3.username, t3.firstname");
$this->db->from("pin as t1");
$this->db->join('(SELECT id, title FROM board) AS t2', 't1.board_id = t2.id', 'left');
$this->db->join('(SELECT id, email, username, firstname FROM user status = 1 AND hidden_profile = 1) AS t3', 't1.user_id = t3.id', 'left');
$this->db->where('t1.status', 1);
$this->db->group_by('t1.user_id');
$this->db->limit($limit, $start);
$query = $this->db->get();
return $query->result_array();