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'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    7676, 7683, 7298, 7297, 5968, 2639, 5113, 
    7295, 7772, 7771, 7294, 7296, 2640, 
    7768, 7759, 7867, 7758, 7868, 2641, 
    7317, 7320, 7321, 2642, 5112, 3141, 
    7318, 3142, 2643, 3143, 7760, 7761, 
    7762, 3144, 5414, 5411, 5413, 5412, 
    5415, 4321, 4320, 4323, 4322, 4324, 
    4325, 4337, 4338, 4340, 4339
  ) 
GROUP BY 
  p.product_id

Query time 0.00051

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.24"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 34,
            "rows_produced_per_join": 3,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "3.65",
              "eval_cost": "0.34",
              "prefix_cost": "3.99",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`dbggbern`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "dbggbern.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.85",
              "eval_cost": "0.02",
              "prefix_cost": "5.18",
              "data_read_per_join": "4"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`dbggbern`.`cscart_discussion`.`object_id` in (7676,7683,7298,7297,5968,2639,5113,7295,7772,7771,7294,7296,2640,7768,7759,7867,7758,7868,2641,7317,7320,7321,2642,5112,3141,7318,3142,2643,3143,7760,7761,7762,3144,5414,5411,5413,5412,5415,4321,4320,4323,4322,4324,4325,4337,4338,4340,4339)) and (`dbggbern`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "dbggbern.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.04",
              "eval_cost": "0.02",
              "prefix_cost": "5.24",
              "data_read_per_join": "888"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}