SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  cscart_discussion 
  INNER JOIN cscart_products as p ON (
    cscart_discussion.object_id = p.product_id
  ) 
  INNER JOIN cscart_discussion_posts as dp ON (
    cscart_discussion.thread_id = dp.thread_id 
    AND cscart_discussion.object_type = 'P' 
    AND cscart_discussion.company_id = 1
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    111, 51212, 144, 51215, 51214, 51213, 
    51444, 51445, 51446, 51447, 51220, 
    51219, 51218, 51217, 51216, 146, 51448, 
    51449, 51231, 51230, 51229, 51228, 
    51227, 51226, 51225, 109, 51243, 51242, 
    51241, 51240, 51239, 36, 51253, 51252, 
    51251, 51250, 51249, 34, 51272, 51271, 
    51270, 51269, 51268, 51267, 51266, 
    145, 51273, 147, 51277, 51276, 51275, 
    51274
  ) 
GROUP BY 
  p.product_id

Query time 0.00050

JSON explain

{
  "query_block": {
    "select_id": 1,
    "message": "no matching row in const table"
  }
}