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 (
    430995, 431087, 431290, 431381, 431004, 
    431231, 431405, 430983, 431071, 431497, 
    430762, 431274, 431411, 431495, 431017, 
    431175, 431182, 431434, 431465, 430686, 
    430974, 431045, 431403, 430755, 430765, 
    430785, 431001, 431494, 430965, 431025, 
    431457, 381710, 430963, 430966, 431002, 
    431240, 431329, 431343, 381707, 430630, 
    430964, 431243, 431304, 431425, 430752, 
    430773, 431284, 431394, 430711, 430961, 
    431328, 431505, 431235, 431362, 431408, 
    431422, 431498, 430971, 431079, 431355, 
    431383, 431015, 431152, 431260, 431283, 
    431407, 430669, 430700, 430705, 430759, 
    431105, 431488, 430943, 430978, 431018, 
    431057, 431069, 431340, 431352, 431409, 
    430944, 431217, 431337, 431419, 435510, 
    430929, 431030, 431311, 430717, 430781, 
    430938, 430973, 431346, 431427, 387046, 
    430956, 431302, 431415, 431482, 431500
  ) 
GROUP BY 
  p.product_id

Query time 0.00157

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 (430995,431087,431290,431381,431004,431231,431405,430983,431071,431497,430762,431274,431411,431495,431017,431175,431182,431434,431465,430686,430974,431045,431403,430755,430765,430785,431001,431494,430965,431025,431457,381710,430963,430966,431002,431240,431329,431343,381707,430630,430964,431243,431304,431425,430752,430773,431284,431394,430711,430961,431328,431505,431235,431362,431408,431422,431498,430971,431079,431355,431383,431015,431152,431260,431283,431407,430669,430700,430705,430759,431105,431488,430943,430978,431018,431057,431069,431340,431352,431409,430944,431217,431337,431419,435510,430929,431030,431311,430717,430781,430938,430973,431346,431427,387046,430956,431302,431415,431482,431500)) 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"
            ]
          }
        }
      ]
    }
  }
}