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 (
    431037, 431056, 431262, 431012, 431186, 
    431392, 430793, 430799, 431034, 431334, 
    430683, 431289, 431301, 431354, 430877, 
    430879, 431166, 431244, 431344, 430920, 
    430926, 430946, 431000, 430659, 430706, 
    430997, 431020, 431218, 431291, 431406, 
    430741, 430864, 431021, 431248, 431309, 
    431322, 431368, 431487, 339481, 431349, 
    430761, 430792, 430858, 430882, 430949, 
    431038, 431101, 431252, 431280, 431469, 
    430830, 431230, 431376, 431420, 431429, 
    430740, 430919, 430975, 431295, 431303, 
    431360, 431464, 430642, 430671, 431410, 
    431424, 431470, 431047, 431055, 431350, 
    431474, 430643, 430739, 430867, 430957, 
    430982, 431060, 431279, 430679, 430814, 
    431211, 431237, 431341, 430735, 430948, 
    431149, 431246, 392168, 430798, 430667, 
    430734, 430764, 431234, 431287, 430934, 
    431010, 431273, 431324, 430718, 430754
  ) 
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 (431037,431056,431262,431012,431186,431392,430793,430799,431034,431334,430683,431289,431301,431354,430877,430879,431166,431244,431344,430920,430926,430946,431000,430659,430706,430997,431020,431218,431291,431406,430741,430864,431021,431248,431309,431322,431368,431487,339481,431349,430761,430792,430858,430882,430949,431038,431101,431252,431280,431469,430830,431230,431376,431420,431429,430740,430919,430975,431295,431303,431360,431464,430642,430671,431410,431424,431470,431047,431055,431350,431474,430643,430739,430867,430957,430982,431060,431279,430679,430814,431211,431237,431341,430735,430948,431149,431246,392168,430798,430667,430734,430764,431234,431287,430934,431010,431273,431324,430718,430754)) 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"
            ]
          }
        }
      ]
    }
  }
}