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, 431290, 431004, 431087, 430983, 
    431071, 431231, 431405, 431497, 431381, 
    431411, 431017, 431182, 431274, 431434, 
    430686, 430974, 431045, 431175, 430765, 
    430785, 431001, 431494, 431495, 430762, 
    430965, 431403, 431465, 381710, 430963, 
    431002, 431025, 431240, 431329, 381707, 
    430755, 430964, 430966, 431457, 430630, 
    430752, 430773, 431304, 431284, 431425, 
    431328, 430711, 430961, 431235, 431343, 
    431362, 431422, 431498, 431505, 431079, 
    431355, 431394, 431408, 431243, 431260, 
    431383, 431407, 430759, 431015, 431283, 
    431488, 430669, 430700, 430943, 430971, 
    431105, 431152, 431340, 431352, 431409, 
    430944, 431018, 431057, 431069, 431217, 
    431337, 430705, 430929, 430978, 431030, 
    431419, 430717, 430781, 431311, 431346, 
    431427, 435510, 430938, 430956, 431302, 
    431482, 431500, 431415, 431453, 430732
  ) 
GROUP BY 
  p.product_id

Query time 0.00099

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 (430995,431290,431004,431087,430983,431071,431231,431405,431497,431381,431411,431017,431182,431274,431434,430686,430974,431045,431175,430765,430785,431001,431494,431495,430762,430965,431403,431465,381710,430963,431002,431025,431240,431329,381707,430755,430964,430966,431457,430630,430752,430773,431304,431284,431425,431328,430711,430961,431235,431343,431362,431422,431498,431505,431079,431355,431394,431408,431243,431260,431383,431407,430759,431015,431283,431488,430669,430700,430943,430971,431105,431152,431340,431352,431409,430944,431018,431057,431069,431217,431337,430705,430929,430978,431030,431419,430717,430781,431311,431346,431427,435510,430938,430956,431302,431482,431500,431415,431453,430732)) 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"
            ]
          }
        }
      ]
    }
  }
}