SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  netting_discussion 
  INNER JOIN netting_products as p ON (
    netting_discussion.object_id = p.product_id
  ) 
  INNER JOIN netting_discussion_posts as dp ON (
    netting_discussion.thread_id = dp.thread_id 
    AND netting_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (297, 298) 
GROUP BY 
  p.product_id

Query time 0.00061

JSON explain

{
  "query_block": {
    "select_id": 1,
    "filesort": {
      "sort_key": "p.product_id",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "p",
              "access_type": "range",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "3",
              "used_key_parts": ["product_id"],
              "rows": 2,
              "filtered": 100,
              "attached_condition": "p.product_id in (297,298)",
              "using_index": true
            }
          },
          {
            "table": {
              "table_name": "netting_discussion",
              "access_type": "eq_ref",
              "possible_keys": ["PRIMARY", "object_id"],
              "key": "object_id",
              "key_length": "6",
              "used_key_parts": ["object_id", "object_type"],
              "ref": ["pucuda_cscart.p.product_id", "const"],
              "rows": 1,
              "filtered": 100,
              "index_condition": "netting_discussion.object_type = 'P'"
            }
          },
          {
            "block-nl-join": {
              "table": {
                "table_name": "dp",
                "access_type": "ALL",
                "possible_keys": ["thread_id", "thread_id_2"],
                "rows": 4,
                "filtered": 100,
                "attached_condition": "dp.`status` = 'A'"
              },
              "buffer_type": "flat",
              "buffer_size": "172",
              "join_type": "BNL",
              "attached_condition": "dp.thread_id = netting_discussion.thread_id"
            }
          }
        ]
      }
    }
  }
}