SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  ushop_discussion 
  INNER JOIN ushop_products as p ON (
    ushop_discussion.object_id = p.product_id
  ) 
  INNER JOIN ushop_discussion_posts as dp ON (
    ushop_discussion.thread_id = dp.thread_id 
    AND ushop_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    517, 525, 511, 489, 516, 483, 513, 496, 
    479, 519, 509, 510, 488, 512, 478, 490, 
    503, 482, 514, 492, 507, 521, 520, 502, 
    508, 480, 515, 506, 518, 497, 505, 500, 
    495
  ) 
GROUP BY 
  p.product_id

Query time 0.00196

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.89"
    },
    "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": 10,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.42",
              "eval_cost": "0.10",
              "prefix_cost": "1.52",
              "data_read_per_join": "448"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`marketplace`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "ushop_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "marketplace.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.01",
              "prefix_cost": "1.87",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`marketplace`.`ushop_discussion`.`object_id` in (517,525,511,489,516,483,513,496,479,519,509,510,488,512,478,490,503,482,514,492,507,521,520,502,508,480,515,506,518,497,505,500,495)) and (`marketplace`.`ushop_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": [
              "marketplace.ushop_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.01",
              "eval_cost": "0.01",
              "prefix_cost": "1.89",
              "data_read_per_join": "220"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}