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 (
    51297, 51296, 139, 51300, 51299, 51298, 
    51304, 51303, 51302, 51301, 138, 51443, 
    51309, 51308, 51307, 51306, 51305, 
    129, 54, 51313, 51312, 51314, 51311, 
    51310, 51361, 30, 51362, 51363, 51364, 
    51365, 31, 51366, 51367, 51368, 51369, 
    46, 51370, 51371, 51372, 51373, 51452, 
    51453, 42, 51376, 51377, 51378, 51379, 
    51380, 88, 51395, 51396, 51397, 51555, 
    51556
  ) 
GROUP BY 
  p.product_id

Query time 0.00062

JSON explain

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