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 (
    908, 3090, 2752, 1143, 2559, 2822, 2253, 
    2751, 2255, 2927, 2729, 3140, 1542, 
    3026, 853, 1540, 1544, 2736, 4887, 2251, 
    2252, 2254, 2869, 3174, 2877, 2788, 
    855, 2795, 5130, 2936, 2792, 3159, 3021, 
    3373, 3011, 2760, 3060, 2880, 2268, 
    3014, 3404, 3366, 3160, 3023, 3099, 
    2800, 2860, 3102, 2775, 2767, 2562, 
    3548, 5151, 2786, 3408, 3744, 2733, 
    3359, 5126, 5133, 2759, 5137, 3375, 
    3517, 5143, 8696, 8243, 8617, 8570, 
    8703, 7873, 7871, 7874, 9872, 10455, 
    8709, 8885, 7879, 8614, 8757
  ) 
GROUP BY 
  p.product_id

Query time 0.00097

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.31"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 13,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.70",
              "eval_cost": "0.13",
              "prefix_cost": "1.83",
              "data_read_per_join": "582"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.33",
              "eval_cost": "0.01",
              "prefix_cost": "2.28",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_id` in (908,3090,2752,1143,2559,2822,2253,2751,2255,2927,2729,3140,1542,3026,853,1540,1544,2736,4887,2251,2252,2254,2869,3174,2877,2788,855,2795,5130,2936,2792,3159,3021,3373,3011,2760,3060,2880,2268,3014,3404,3366,3160,3023,3099,2800,2860,3102,2775,2767,2562,3548,5151,2786,3408,3744,2733,3359,5126,5133,2759,5137,3375,3517,5143,8696,8243,8617,8570,8703,7873,7871,7874,9872,10455,8709,8885,7879,8614,8757)) and (`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "softwarepirmam_hewadelivard_cscart_4.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.02",
              "eval_cost": "0.01",
              "prefix_cost": "2.31",
              "data_read_per_join": "294"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}