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 (
    350896, 350905, 357647, 359189, 350826, 
    350903, 351714, 358176, 357025, 357062, 
    357980, 359390, 357420, 358077, 350743, 
    356989, 359337, 350871, 355212, 357949, 
    357681, 359278, 357407, 358129, 359255, 
    357009, 357921, 359233, 350910, 355208, 
    350857, 357068, 357235, 357238, 358246, 
    356173, 357244, 359152, 352295, 357754, 
    351679, 357209, 358131, 355239, 359231, 
    355183, 357740, 357823, 351174, 351186, 
    357951, 359150, 357698, 358208, 359321, 
    359345, 353962, 354943, 357943, 359341, 
    352275, 354913, 357825, 359141, 359168, 
    359334, 350673, 350835, 358149, 353875, 
    354591, 354935, 354966, 357393, 357760, 
    359263, 357012, 357831, 357953, 359376, 
    357404, 357854, 351681, 355359, 357039, 
    357880, 358023, 358218, 350678, 357848, 
    358249, 351664, 356175, 358251, 356978, 
    355311, 357124, 353975, 355363, 358079
  ) 
GROUP BY 
  p.product_id

Query time 0.00164

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 (350896,350905,357647,359189,350826,350903,351714,358176,357025,357062,357980,359390,357420,358077,350743,356989,359337,350871,355212,357949,357681,359278,357407,358129,359255,357009,357921,359233,350910,355208,350857,357068,357235,357238,358246,356173,357244,359152,352295,357754,351679,357209,358131,355239,359231,355183,357740,357823,351174,351186,357951,359150,357698,358208,359321,359345,353962,354943,357943,359341,352275,354913,357825,359141,359168,359334,350673,350835,358149,353875,354591,354935,354966,357393,357760,359263,357012,357831,357953,359376,357404,357854,351681,355359,357039,357880,358023,358218,350678,357848,358249,351664,356175,358251,356978,355311,357124,353975,355363,358079)) 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"
            ]
          }
        }
      ]
    }
  }
}