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 (
    2698, 2340, 3834, 3306, 5848, 2352, 2431, 
    5114, 2922, 5798, 5801, 5829, 3301, 
    5852, 5928, 5847, 2341, 5794, 2702, 
    3147, 3520, 3529, 3146, 3809, 3148, 
    3215, 3213, 3552, 3149, 3157, 5791, 
    3845, 3150, 2228, 3664, 2230, 2655, 
    5890, 2226, 3843, 3840, 5891, 2719, 
    5111, 3385, 3831, 3289, 3382, 3222, 
    3522, 5804, 5113, 3537, 3387, 3388, 
    3528, 3819, 3309, 2918, 3384, 3532, 
    2720, 3807, 2727, 2229, 3550, 3220, 
    5107, 6112, 2429, 3805, 3524, 2921, 
    3668, 3702, 3670, 3684, 3682, 3230, 
    3303
  ) 
GROUP BY 
  p.product_id

Query time 0.00194

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 (2698,2340,3834,3306,5848,2352,2431,5114,2922,5798,5801,5829,3301,5852,5928,5847,2341,5794,2702,3147,3520,3529,3146,3809,3148,3215,3213,3552,3149,3157,5791,3845,3150,2228,3664,2230,2655,5890,2226,3843,3840,5891,2719,5111,3385,3831,3289,3382,3222,3522,5804,5113,3537,3387,3388,3528,3819,3309,2918,3384,3532,2720,3807,2727,2229,3550,3220,5107,6112,2429,3805,3524,2921,3668,3702,3670,3684,3682,3230,3303)) 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"
            ]
          }
        }
      ]
    }
  }
}