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 (
    51514, 51538, 51539, 51468, 51476, 51491, 
    51513, 51474, 51503, 51520, 51541, 
    51462, 51502, 51522, 51499, 51465, 
    51509, 51515, 51466, 51485, 51518, 
    51550, 51519, 51524, 51528, 51508, 
    51527, 51543, 51482, 51504, 51542, 
    51512, 51493, 51511, 51505, 51046, 
    51439, 51544, 51054, 51546, 51548, 
    51438, 51535
  ) 
GROUP BY 
  p.product_id

Query time 0.00042

JSON explain

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