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 (
    431055, 430742, 430675, 430800, 430805, 
    430839, 430855, 430863, 431428, 430953, 
    430979, 431005, 430778, 431244, 431262, 
    431336, 430688, 430755, 430852, 430874, 
    430878, 430894, 430917, 430931, 430952, 
    430995, 431000, 431177, 431368, 430713, 
    430826, 430827, 431027, 431028, 431101, 
    431184, 431504, 430780, 430856, 430897, 
    430991, 431342, 431358, 431451, 431469, 
    431234, 431357, 430864, 430694, 430840, 
    431420, 430671, 430871, 430977, 431010, 
    431344, 430702, 430706, 430888, 430918, 
    430666, 430667, 431129, 431297, 430895, 
    430972, 430990, 431289, 430982, 430997, 
    431025, 431037, 431165, 431295, 430692, 
    430933, 431007, 431308, 431381, 430723, 
    430804, 430720, 430946, 431149, 431457, 
    430674, 430679, 431237, 431280, 431354, 
    431376, 431424, 431470, 431047, 431087, 
    431288, 431355, 430726, 430886, 431138
  ) 
GROUP BY 
  p.product_id

Query time 0.00097

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 (431055,430742,430675,430800,430805,430839,430855,430863,431428,430953,430979,431005,430778,431244,431262,431336,430688,430755,430852,430874,430878,430894,430917,430931,430952,430995,431000,431177,431368,430713,430826,430827,431027,431028,431101,431184,431504,430780,430856,430897,430991,431342,431358,431451,431469,431234,431357,430864,430694,430840,431420,430671,430871,430977,431010,431344,430702,430706,430888,430918,430666,430667,431129,431297,430895,430972,430990,431289,430982,430997,431025,431037,431165,431295,430692,430933,431007,431308,431381,430723,430804,430720,430946,431149,431457,430674,430679,431237,431280,431354,431376,431424,431470,431047,431087,431288,431355,430726,430886,431138)) 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"
            ]
          }
        }
      ]
    }
  }
}