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 (
    20673, 20947, 22253, 20168, 19430, 19431, 
    19432, 19438, 19439, 19440, 19443, 
    19444, 19445, 19760, 19761, 19814, 
    19815, 19816, 19817, 19819, 19861, 
    20674, 22462, 21341, 19934, 19138, 
    19433, 19434, 19435, 19436, 19437, 
    19441, 19442, 19759, 19818, 19932
  ) 
GROUP BY 
  p.product_id

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "p",
          "access_type": "range",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["product_id"],
          "rows": 36,
          "filtered": 100,
          "attached_condition": "p.product_id in (20673,20947,22253,20168,19430,19431,19432,19438,19439,19440,19443,19444,19445,19760,19761,19814,19815,19816,19817,19819,19861,20674,22462,21341,19934,19138,19433,19434,19435,19436,19437,19441,19442,19759,19818,19932)",
          "using_index": true
        }
      },
      {
        "table": {
          "table_name": "cscart_discussion",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY", "object_id", "idx_company_id"],
          "key": "object_id",
          "key_length": "10",
          "used_key_parts": ["object_id", "object_type", "company_id"],
          "ref": ["lillyshop_lillyshop.p.product_id", "const", "const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "cscart_discussion.object_type = 'P'"
        }
      },
      {
        "table": {
          "table_name": "dp",
          "access_type": "ref",
          "possible_keys": ["thread_id", "thread_id_2"],
          "key": "thread_id_2",
          "key_length": "6",
          "used_key_parts": ["thread_id", "status"],
          "ref": ["lillyshop_lillyshop.cscart_discussion.thread_id", "const"],
          "rows": 2,
          "filtered": 100,
          "index_condition": "dp.`status` = 'A'"
        }
      }
    ]
  }
}