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 (
    128, 51118, 51117, 51116, 51115, 51114, 
    51113, 51165, 51164, 51163, 51162, 
    51161, 52, 51183, 20, 51188, 51187, 
    51186, 51185, 51184, 51201, 51200, 
    51199, 51198, 51197, 51196, 51195, 
    131, 51231, 51230, 51229, 51228, 51227, 
    51226, 51225, 109, 51243, 51242, 51241, 
    51240, 51239, 36, 51253, 51252, 51251, 
    51250, 51249, 34, 51265, 51264, 51263, 
    51262, 51261, 51260, 51259, 110, 51273, 
    147, 51277, 51276, 51275, 51274, 51346, 
    118, 114, 84, 51352, 51351, 96
  ) 
GROUP BY 
  p.product_id

Query time 0.00078

JSON explain

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