SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  tmall_discussion 
  INNER JOIN tmall_products as p ON (
    tmall_discussion.object_id = p.product_id
  ) 
  INNER JOIN tmall_discussion_posts as dp ON (
    tmall_discussion.thread_id = dp.thread_id 
    AND tmall_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (102) 
GROUP BY 
  p.product_id

Query time 0.00753

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}