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 (
    339487, 339682, 339613, 339639, 339555, 
    339480, 339560, 271697, 339579, 339854, 
    339680, 339559, 339788, 339850, 339772, 
    339475, 339815, 339785, 339561, 339566, 
    339766, 339558, 339568, 339688, 339661, 
    339563, 339819, 339837, 339640, 339585, 
    339790, 339598, 339634, 381694, 339578, 
    339546, 339629, 392159, 339672, 381692, 
    339844, 339641, 339624, 339832, 339627, 
    339840, 381689, 339687, 381685, 220162, 
    339842, 339612, 339800, 339642, 339827, 
    265036, 339764, 339605, 339564, 220170, 
    431190, 339690, 339462, 381682, 430574, 
    339562, 339663, 339793, 339554, 339700, 
    271696, 392172, 271695, 431092, 339792, 
    381688, 431106, 435505, 339811, 381693, 
    381691, 220169, 431147, 339801, 339459, 
    339590, 339473, 339592, 430849, 220166, 
    339799, 339679, 381690, 339791, 339795, 
    339660, 339623, 339803, 339813, 339659
  ) 
GROUP BY 
  p.product_id

Query time 0.00098

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`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": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (339487,339682,339613,339639,339555,339480,339560,271697,339579,339854,339680,339559,339788,339850,339772,339475,339815,339785,339561,339566,339766,339558,339568,339688,339661,339563,339819,339837,339640,339585,339790,339598,339634,381694,339578,339546,339629,392159,339672,381692,339844,339641,339624,339832,339627,339840,381689,339687,381685,220162,339842,339612,339800,339642,339827,265036,339764,339605,339564,220170,431190,339690,339462,381682,430574,339562,339663,339793,339554,339700,271696,392172,271695,431092,339792,381688,431106,435505,339811,381693,381691,220169,431147,339801,339459,339590,339473,339592,430849,220166,339799,339679,381690,339791,339795,339660,339623,339803,339813,339659)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}