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 (
    430732, 430833, 430942, 430985, 431076, 
    431466, 431480, 339768, 387046, 430750, 
    431210, 431219, 431330, 431438, 431452, 
    430719, 431380, 430724, 430987, 431181, 
    431222, 431285, 431377, 431481, 431239, 
    431353, 431387, 381709, 430693, 431229, 
    431241, 431412, 431447, 430632, 430727, 
    431292, 431384, 431385, 431440, 431502, 
    431208, 431236, 430733, 431188, 431281, 
    430721, 430682, 431259, 431224, 431335, 
    431361, 430699, 431421, 430701, 430955, 
    431249, 431253, 431332, 431458, 430951, 
    431040, 431081, 431172, 431402, 430731, 
    430757, 431431, 430945, 431009, 431143, 
    431215, 431263, 431298, 431468, 381715, 
    430736, 431170, 431442, 431455, 431472, 
    381706, 431364, 431401, 431414, 431478, 
    430970, 431180, 431454, 381708, 430941, 
    430954, 431233, 431369, 431378, 431446, 
    430725, 431014, 431213, 431225, 431300
  ) 
GROUP BY 
  p.product_id

Query time 0.00093

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 (430732,430833,430942,430985,431076,431466,431480,339768,387046,430750,431210,431219,431330,431438,431452,430719,431380,430724,430987,431181,431222,431285,431377,431481,431239,431353,431387,381709,430693,431229,431241,431412,431447,430632,430727,431292,431384,431385,431440,431502,431208,431236,430733,431188,431281,430721,430682,431259,431224,431335,431361,430699,431421,430701,430955,431249,431253,431332,431458,430951,431040,431081,431172,431402,430731,430757,431431,430945,431009,431143,431215,431263,431298,431468,381715,430736,431170,431442,431455,431472,381706,431364,431401,431414,431478,430970,431180,431454,381708,430941,430954,431233,431369,431378,431446,430725,431014,431213,431225,431300)) 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"
            ]
          }
        }
      ]
    }
  }
}