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 (
    350511, 355422, 350579, 355405, 350381, 
    350420, 350306, 354322, 350347, 354323, 
    350342, 350368, 355618, 350283, 355620, 
    354738, 350435, 355619, 350504, 355446, 
    350226, 350335, 350442, 350448, 350468, 
    355421, 354526, 350345, 350456, 356842, 
    350385, 350513, 355389, 350282, 350519, 
    355408, 355412, 350417, 358345, 353702, 
    350337, 350412, 358351, 350362, 355406, 
    350462, 353716, 350583, 354523, 350375, 
    358343, 350352, 353726, 350361, 353701, 
    350268, 350377, 350410, 350411, 354743, 
    355429, 355425, 350346, 355418, 358333, 
    350313, 350373, 353712, 353723, 355415, 
    355417, 355438, 354524, 350379, 350380, 
    350388, 355396, 355409, 355439, 355420, 
    350382, 350403, 354525, 358348, 358349, 
    350378, 355395, 353105, 353813, 350589, 
    355411, 358350, 350393, 350311, 355390, 
    358353, 358347, 350384, 350310, 350421
  ) 
GROUP BY 
  p.product_id

Query time 0.00092

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`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": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (350511,355422,350579,355405,350381,350420,350306,354322,350347,354323,350342,350368,355618,350283,355620,354738,350435,355619,350504,355446,350226,350335,350442,350448,350468,355421,354526,350345,350456,356842,350385,350513,355389,350282,350519,355408,355412,350417,358345,353702,350337,350412,358351,350362,355406,350462,353716,350583,354523,350375,358343,350352,353726,350361,353701,350268,350377,350410,350411,354743,355429,355425,350346,355418,358333,350313,350373,353712,353723,355415,355417,355438,354524,350379,350380,350388,355396,355409,355439,355420,350382,350403,354525,358348,358349,350378,355395,353105,353813,350589,355411,358350,350393,350311,355390,358353,358347,350384,350310,350421)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}