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 (
    5504, 5384, 5506, 6018, 6749, 5385, 5464, 
    6185, 6735, 6114, 5647, 6713, 6146, 
    6187, 6742, 5651, 6086, 6299, 6307, 
    6371, 5508, 6081, 8812, 6740, 6705, 
    6144, 6254, 6433, 6755, 6228, 6192, 
    6227, 6356, 6083, 6230, 6710, 6275, 
    6277, 6393, 6426, 6258, 6428, 8800, 
    6255, 6394, 6229, 6256, 8576, 6189, 
    6257, 6259, 6145, 6332, 6362, 8577, 
    6278, 5644, 6226, 6279, 6392, 6297, 
    6354, 6331, 6337, 6395, 6423, 6427, 
    6364, 8569, 6190, 6276, 6303, 6359, 
    6429, 6280, 6260, 6305, 6330, 6434, 
    6355
  ) 
GROUP BY 
  p.product_id

Query time 0.00102

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 (5504,5384,5506,6018,6749,5385,5464,6185,6735,6114,5647,6713,6146,6187,6742,5651,6086,6299,6307,6371,5508,6081,8812,6740,6705,6144,6254,6433,6755,6228,6192,6227,6356,6083,6230,6710,6275,6277,6393,6426,6258,6428,8800,6255,6394,6229,6256,8576,6189,6257,6259,6145,6332,6362,8577,6278,5644,6226,6279,6392,6297,6354,6331,6337,6395,6423,6427,6364,8569,6190,6276,6303,6359,6429,6280,6260,6305,6330,6434,6355)) 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"
            ]
          }
        }
      ]
    }
  }
}