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 (
    51194, 51193, 51192, 51191, 51190, 51189, 
    19, 111, 51212, 144, 51215, 51214, 51213, 
    51444, 51445, 51446, 51447, 51220, 
    51219, 51218, 51217, 51216, 146, 51448, 
    51449, 9, 59, 60, 13, 51347, 85, 61, 51350, 
    123, 15, 95
  ) 
GROUP BY 
  p.product_id

Query time 0.00042

JSON explain

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