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 (
    1, 228, 8, 9, 10, 71, 4, 1549, 5851, 1550, 
    1548, 5737, 5995, 5728, 5879, 5968, 
    5920, 1551, 5876, 5902, 5860, 1546, 
    5846, 5897, 5880, 5782, 5985, 5784, 
    1547, 5767, 5780, 5844, 5764, 5872, 
    5971, 5841, 5768, 5835, 5910, 5873, 
    5853, 5901, 8266, 5763, 5855, 5921, 
    5883, 5904, 8265, 5769, 5772, 8267, 
    5886, 5906, 5996, 5783, 5913, 5781, 
    5842, 5840, 5986, 8264, 5770, 5916, 
    5865, 5867, 5907, 5771, 5903, 5776, 
    5889, 5874, 5834, 5878, 5875, 5877, 
    5905, 5963, 5863, 5909
  ) 
GROUP BY 
  p.product_id

Query time 0.00119

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 (1,228,8,9,10,71,4,1549,5851,1550,1548,5737,5995,5728,5879,5968,5920,1551,5876,5902,5860,1546,5846,5897,5880,5782,5985,5784,1547,5767,5780,5844,5764,5872,5971,5841,5768,5835,5910,5873,5853,5901,8266,5763,5855,5921,5883,5904,8265,5769,5772,8267,5886,5906,5996,5783,5913,5781,5842,5840,5986,8264,5770,5916,5865,5867,5907,5771,5903,5776,5889,5874,5834,5878,5875,5877,5905,5963,5863,5909)) 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"
            ]
          }
        }
      ]
    }
  }
}