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 (
    339707, 392169, 339656, 430908, 339769, 
    339807, 431115, 431133, 339829, 339758, 
    339677, 339809, 431050, 339729, 381702, 
    431150, 430747, 431026, 339648, 431158, 
    430681, 339851, 431195, 339479, 430748, 
    339847, 381696, 431122, 339654, 339712, 
    339746, 430907, 431102, 430924, 430998, 
    387047, 339704, 339735, 430648, 339664, 
    430696, 431201, 430771, 339760, 430916, 
    339485, 339817, 430824, 430915, 339720, 
    430636, 430638, 430823, 431139, 339812, 
    430743, 430857, 430646, 430813, 430935, 
    430803, 431120, 430821, 339714, 339835, 
    430831, 431145, 431151, 430665, 339668, 
    381698, 339715, 339761, 430649, 430751, 
    387049, 339671, 339820, 381699, 430647, 
    430848, 431127, 431109, 430766, 339694, 
    339650, 430903, 339593, 431114, 430660, 
    430817, 431097, 431163, 431194, 339849, 
    431103, 430902, 430912, 339755, 430807
  ) 
GROUP BY 
  p.product_id

Query time 0.00119

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 (339707,392169,339656,430908,339769,339807,431115,431133,339829,339758,339677,339809,431050,339729,381702,431150,430747,431026,339648,431158,430681,339851,431195,339479,430748,339847,381696,431122,339654,339712,339746,430907,431102,430924,430998,387047,339704,339735,430648,339664,430696,431201,430771,339760,430916,339485,339817,430824,430915,339720,430636,430638,430823,431139,339812,430743,430857,430646,430813,430935,430803,431120,430821,339714,339835,430831,431145,431151,430665,339668,381698,339715,339761,430649,430751,387049,339671,339820,381699,430647,430848,431127,431109,430766,339694,339650,430903,339593,431114,430660,430817,431097,431163,431194,339849,431103,430902,430912,339755,430807)) 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"
            ]
          }
        }
      ]
    }
  }
}