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 (
    908, 134, 955, 942, 941, 446, 953, 952, 
    951, 949, 948, 947, 946, 945, 944, 994, 
    996, 493, 985, 986, 988, 990, 991, 993, 
    1128, 1127, 1126, 1125, 1124, 1123, 
    1139, 1138, 1137, 1136, 1135, 1134, 
    1133, 1132, 1131, 1130, 1129, 827, 1142, 
    853, 1146, 1145, 1144, 1143, 856, 1322, 
    1262, 1317, 1318, 1319, 1320, 1321, 
    1256, 1395, 1396, 1397, 1399, 1406, 
    1407, 1408, 1409, 1410, 1411, 1412, 
    1273, 1425, 1427, 1428, 1429, 1431, 
    1266, 1442, 1447, 1263, 1458, 1284, 
    1475, 1476, 1477, 1274, 1483, 1484, 
    1485, 1486, 1487, 1540, 1541, 1542, 
    1543, 1538, 8496, 1517, 8498, 8497, 
    1532, 8502, 8501, 8499, 1528, 8505, 
    8503
  ) 
GROUP BY 
  p.product_id

Query time 0.00115

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 (908,134,955,942,941,446,953,952,951,949,948,947,946,945,944,994,996,493,985,986,988,990,991,993,1128,1127,1126,1125,1124,1123,1139,1138,1137,1136,1135,1134,1133,1132,1131,1130,1129,827,1142,853,1146,1145,1144,1143,856,1322,1262,1317,1318,1319,1320,1321,1256,1395,1396,1397,1399,1406,1407,1408,1409,1410,1411,1412,1273,1425,1427,1428,1429,1431,1266,1442,1447,1263,1458,1284,1475,1476,1477,1274,1483,1484,1485,1486,1487,1540,1541,1542,1543,1538,8496,1517,8498,8497,1532,8502,8501,8499,1528,8505,8503)) 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"
            ]
          }
        }
      ]
    }
  }
}