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 (
    6903, 7817, 7805, 7819, 7803, 7824, 7905, 
    8336, 7815, 6941, 7121, 7086, 7714, 
    7176, 6938, 6911, 6994, 7084, 6993, 
    6991, 7027, 6939, 7054, 6940, 7169, 
    7120, 7712, 7138, 7095, 7167, 7141, 
    7695, 7031, 7089, 6992, 6892, 7125, 
    6880, 6996, 7173, 7210, 7711, 7715, 
    7792, 7170, 7174, 7087, 7177, 6989, 
    6995, 6997, 7142, 7171, 7172, 7132, 
    7795, 7709, 7806, 7918, 7119, 7696, 
    7793, 7903, 7929, 7931, 7791, 7814, 
    7818, 7822, 7902, 7925, 7928, 7672, 
    7697, 7713, 7790, 7808, 7820, 7821, 
    7831
  ) 
GROUP BY 
  p.product_id

Query time 0.00118

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 (6903,7817,7805,7819,7803,7824,7905,8336,7815,6941,7121,7086,7714,7176,6938,6911,6994,7084,6993,6991,7027,6939,7054,6940,7169,7120,7712,7138,7095,7167,7141,7695,7031,7089,6992,6892,7125,6880,6996,7173,7210,7711,7715,7792,7170,7174,7087,7177,6989,6995,6997,7142,7171,7172,7132,7795,7709,7806,7918,7119,7696,7793,7903,7929,7931,7791,7814,7818,7822,7902,7925,7928,7672,7697,7713,7790,7808,7820,7821,7831)) 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"
            ]
          }
        }
      ]
    }
  }
}