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 (
    4219, 1273, 5830, 2340, 4526, 1559, 2502, 
    5887, 2187, 2473, 2508, 4463, 1532, 
    1568, 2565, 2300, 2580, 3354, 1591, 
    2638, 4215, 3155, 3834, 2301, 4294, 
    4316, 2737, 2550, 1590, 2275, 2559, 
    1564, 3308, 1579, 2363, 4126, 4796, 
    1567, 4562, 1583, 5892, 3306, 1578, 
    5625, 5848, 2845, 1585, 1573, 1572, 
    1574, 8499, 2599, 1549, 2822, 8502, 
    8501, 1592, 4170, 2640, 2399, 2646, 
    1576, 1665, 1586, 3397, 2352, 2503, 
    2602, 2459, 1593, 3363, 2431, 5899, 
    2201, 5934, 5939, 3929, 5918, 3841, 
    4172
  ) 
GROUP BY 
  p.product_id

Query time 0.00124

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 (4219,1273,5830,2340,4526,1559,2502,5887,2187,2473,2508,4463,1532,1568,2565,2300,2580,3354,1591,2638,4215,3155,3834,2301,4294,4316,2737,2550,1590,2275,2559,1564,3308,1579,2363,4126,4796,1567,4562,1583,5892,3306,1578,5625,5848,2845,1585,1573,1572,1574,8499,2599,1549,2822,8502,8501,1592,4170,2640,2399,2646,1576,1665,1586,3397,2352,2503,2602,2459,1593,3363,2431,5899,2201,5934,5939,3929,5918,3841,4172)) 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"
            ]
          }
        }
      ]
    }
  }
}