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 (
    1003, 1001, 1000, 508, 1139, 1138, 1137, 
    1136, 1135, 1134, 1133, 1132, 1131, 
    1130, 1129, 827, 1406, 1407, 1408, 1409, 
    1410, 1411, 1412, 1266, 1442, 1447, 
    2490, 2491, 2492, 2493, 2494, 2495, 
    2496, 2497, 2524, 2525, 2526, 2527, 
    2528, 2529, 2530, 2531, 2698, 3090, 
    3091, 3092, 3093, 3094, 3974, 3977, 
    3976, 3975, 3973, 3972, 3978, 3979, 
    4022, 4021, 4020, 4420, 4423, 4426, 
    4416, 5618, 5619, 5620, 5621, 5696, 
    5697, 5698, 5699, 5721, 5722, 5723, 
    5724, 5725, 5726
  ) 
GROUP BY 
  p.product_id

Query time 0.00081

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 (1003,1001,1000,508,1139,1138,1137,1136,1135,1134,1133,1132,1131,1130,1129,827,1406,1407,1408,1409,1410,1411,1412,1266,1442,1447,2490,2491,2492,2493,2494,2495,2496,2497,2524,2525,2526,2527,2528,2529,2530,2531,2698,3090,3091,3092,3093,3094,3974,3977,3976,3975,3973,3972,3978,3979,4022,4021,4020,4420,4423,4426,4416,5618,5619,5620,5621,5696,5697,5698,5699,5721,5722,5723,5724,5725,5726)) 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"
            ]
          }
        }
      ]
    }
  }
}