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 (
    51007, 51004, 51005, 51010, 51008, 51012, 
    51013, 51011, 51015, 51006, 51021, 
    51016, 51024, 51017, 51019, 51018, 
    51022, 51014, 51045, 51025, 51029, 
    51060, 51027, 51063, 51020, 51067, 
    51028, 51031, 51062, 51042, 51030, 
    51032, 51033, 51026, 51035, 51059, 
    51069, 51070, 51038, 51066, 51023, 
    51041, 51056, 51040, 51050, 51065, 
    51039, 51052, 51009, 51034, 51061, 
    51036, 51037, 51068, 51064, 51055, 
    51058, 51071
  ) 
GROUP BY 
  p.product_id

Query time 0.00074

JSON explain

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