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 (
    356409, 350046, 354359, 356853, 352482, 
    353629, 354406, 355426, 355756, 359005, 
    353291, 353422, 353953, 353011, 354242, 
    352265, 353900, 354462, 355261, 350775, 
    353633, 354225, 351736, 354304, 355482, 
    356747, 351040, 354404, 351297, 355509, 
    356071, 349717, 355980, 353297, 352887, 
    357199, 351199, 352517, 352931, 355954, 
    356494, 357133, 357328, 357778, 352087, 
    352829, 354677, 356789, 357912, 350238, 
    354170, 351753, 354341, 355693, 356075, 
    358569, 351755, 352593, 353948, 350359, 
    355361, 356708, 350053, 351003, 353554, 
    353857, 357157, 354199, 353151, 353329, 
    353465, 358545, 351200, 353467, 354580, 
    355358, 358830, 350043, 351556, 355662, 
    354403, 351685, 356222, 358385, 350880, 
    350980, 352330, 352211, 350218, 351737, 
    354906, 356225, 357268, 352091, 354263, 
    355290, 349857, 355259, 357941, 358652
  ) 
GROUP BY 
  p.product_id

Query time 0.00147

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 (356409,350046,354359,356853,352482,353629,354406,355426,355756,359005,353291,353422,353953,353011,354242,352265,353900,354462,355261,350775,353633,354225,351736,354304,355482,356747,351040,354404,351297,355509,356071,349717,355980,353297,352887,357199,351199,352517,352931,355954,356494,357133,357328,357778,352087,352829,354677,356789,357912,350238,354170,351753,354341,355693,356075,358569,351755,352593,353948,350359,355361,356708,350053,351003,353554,353857,357157,354199,353151,353329,353465,358545,351200,353467,354580,355358,358830,350043,351556,355662,354403,351685,356222,358385,350880,350980,352330,352211,350218,351737,354906,356225,357268,352091,354263,355290,349857,355259,357941,358652)) 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"
            ]
          }
        }
      ]
    }
  }
}