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 (
    82, 51132, 51131, 51134, 51133, 51130, 
    41, 51146, 51145, 38, 51148, 51147, 
    51150, 51149, 37, 51169, 51168, 51167, 
    51166, 90, 51171, 51170, 51209, 51208, 
    51207, 51206, 124, 56, 51295, 117, 116, 
    115, 81, 11, 51326, 135, 51357, 51360, 
    51359, 51358, 51361, 30, 51362, 51363, 
    51364, 51365, 140, 51385, 51386, 51387, 
    51388, 51389
  ) 
GROUP BY 
  p.product_id

Query time 0.00049

JSON explain

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