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 (
    6303, 6304, 5801, 5802, 5800, 5799, 5803, 
    5804, 7197, 7195, 7196, 7190, 7189, 
    7194, 7191, 5032, 5033, 7674, 7675, 
    6967, 6966, 6645, 6644, 6647, 6648, 
    4352, 4354, 5391, 5392, 5388, 5390, 
    5389, 5396, 5395, 5394, 5034, 5035, 
    5030, 5031, 5168, 5169, 6982, 6132, 
    7150, 7149, 7148, 7151, 6768
  ) 
GROUP BY 
  p.product_id

Query time 0.00058

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.24"
    },
    "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": 34,
            "rows_produced_per_join": 3,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "3.65",
              "eval_cost": "0.34",
              "prefix_cost": "3.99",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`dbggbern`.`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": [
              "dbggbern.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.85",
              "eval_cost": "0.02",
              "prefix_cost": "5.18",
              "data_read_per_join": "4"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`dbggbern`.`cscart_discussion`.`object_id` in (6303,6304,5801,5802,5800,5799,5803,5804,7197,7195,7196,7190,7189,7194,7191,5032,5033,7674,7675,6967,6966,6645,6644,6647,6648,4352,4354,5391,5392,5388,5390,5389,5396,5395,5394,5034,5035,5030,5031,5168,5169,6982,6132,7150,7149,7148,7151,6768)) and (`dbggbern`.`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": [
              "dbggbern.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.04",
              "eval_cost": "0.02",
              "prefix_cost": "5.24",
              "data_read_per_join": "888"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}