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 (
    350161, 352217, 352313, 352916, 353117, 
    353920, 353966, 355244, 356586, 356830, 
    349822, 349960, 350010, 351150, 352541, 
    352656, 355830, 355889, 357377, 358968, 
    359063, 352213, 352670, 353192, 354043, 
    354087, 354376, 354719, 354785, 355102, 
    356168, 356478, 356594, 359339, 350345, 
    350899, 351493, 351600, 352704, 355147, 
    357295, 358552, 349988, 350852, 351121, 
    351472, 351526, 351529, 352354, 352466, 
    352623, 354176, 354428, 354787, 355600, 
    356733, 356953, 357981, 359154, 350851, 
    351369, 351527, 352230, 352344, 352348, 
    352379, 355300, 356172, 357397, 357758, 
    358517, 352169, 354384, 357514, 357821, 
    358391, 350721, 351725, 352108, 352717, 
    353635, 355438, 355522, 357412, 358048, 
    351394, 351441, 352166, 352337, 353982, 
    354337, 354777, 354839, 355134, 355953, 
    356360, 357002, 357822, 358393, 358892
  ) 
GROUP BY 
  p.product_id

Query time 0.00083

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 (350161,352217,352313,352916,353117,353920,353966,355244,356586,356830,349822,349960,350010,351150,352541,352656,355830,355889,357377,358968,359063,352213,352670,353192,354043,354087,354376,354719,354785,355102,356168,356478,356594,359339,350345,350899,351493,351600,352704,355147,357295,358552,349988,350852,351121,351472,351526,351529,352354,352466,352623,354176,354428,354787,355600,356733,356953,357981,359154,350851,351369,351527,352230,352344,352348,352379,355300,356172,357397,357758,358517,352169,354384,357514,357821,358391,350721,351725,352108,352717,353635,355438,355522,357412,358048,351394,351441,352166,352337,353982,354337,354777,354839,355134,355953,356360,357002,357822,358393,358892)) 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"
            ]
          }
        }
      ]
    }
  }
}