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 (
    2906, 5879, 5523, 6689, 5876, 2908, 5474, 
    5452, 5405, 5404, 5527, 6686, 5494, 
    5593, 5488, 5403, 5492, 5453, 5491, 
    5446, 5536, 5551, 6688, 5553, 5515, 
    5435, 2909, 5454, 5558, 5792, 5449, 
    5556, 5519, 5601, 8723, 5444, 5490, 
    5429, 5497, 5483, 6208, 5530, 5531, 
    5769, 5528, 5537, 5557, 5586, 5430, 
    5448, 5587, 6690, 5790, 5436, 5522, 
    5598, 5517, 5451, 5478, 5442, 5510, 
    5552, 5509, 5535, 6242, 5437, 5485, 
    5479, 5518, 5441, 5434, 5590, 5493, 
    5526, 5484, 6219, 5402, 5482, 5589, 
    5600
  ) 
GROUP BY 
  p.product_id

Query time 0.00190

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 (2906,5879,5523,6689,5876,2908,5474,5452,5405,5404,5527,6686,5494,5593,5488,5403,5492,5453,5491,5446,5536,5551,6688,5553,5515,5435,2909,5454,5558,5792,5449,5556,5519,5601,8723,5444,5490,5429,5497,5483,6208,5530,5531,5769,5528,5537,5557,5586,5430,5448,5587,6690,5790,5436,5522,5598,5517,5451,5478,5442,5510,5552,5509,5535,6242,5437,5485,5479,5518,5441,5434,5590,5493,5526,5484,6219,5402,5482,5589,5600)) 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"
            ]
          }
        }
      ]
    }
  }
}