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 (
    396044, 
    396045, 
    396046, 
    396047, 
    396048, 
    396049, 
    396050, 
    396052, 
    396053, 
    396054, 
    396055, 
    396056, 
    396057, 
    396074, 
    396075, 
    396076, 
    396077, 
    396078, 
    396079, 
    396080, 
    396081, 
    396082, 
    396083, 
    396084, 
    396085, 
    414329, 
    414330, 
    414331, 
    414332, 
    414333, 
    414334, 
    414335, 
    414336, 
    414337, 
    414338, 
    414339, 
    414340, 
    414341, 
    414360, 
    414361, 
    414362, 
    414363, 
    414364, 
    414365, 
    414366, 
    414393, 
    414394, 
    414395, 
    414396, 
    414397, 
    414398, 
    414399, 
    414401, 
    414402, 
    414403, 
    414404, 
    414405, 
    414406, 
    414407, 
    414415, 
    414416, 
    414417, 
    414418, 
    414419, 
    414420, 
    414421, 
    414429, 
    414430, 
    414431, 
    414432, 
    414433, 
    414434, 
    414435, 
    414459, 
    414460, 
    414461, 
    414462, 
    414463, 
    414464, 
    414465, 
    414468, 
    414470, 
    414471, 
    414472, 
    414473, 
    414474, 
    414475, 
    414476, 
    414514, 
    414516, 
    414517, 
    414518, 
    414519, 
    414520, 
    414521, 
    431514, 
    431515, 
    431516, 
    431517, 
    431518, 
    431519, 
    431520, 
    431521, 
    431522, 
    431523, 
    431524, 
    431525, 
    431526, 
    431527, 
    431528, 
    431529, 
    431530, 
    431531, 
    431532, 
    431533, 
    431534, 
    431535, 
    431536, 
    431537, 
    431687, 
    431688, 
    431689, 
    431690, 
    431691, 
    431692, 
    431693, 
    431694, 
    431695, 
    431696, 
    431697, 
    431698, 
    431699, 
    431700, 
    431701, 
    431702, 
    431703, 
    431704, 
    431705, 
    431706, 
    431707, 
    431708, 
    431709, 
    431710, 
    431711, 
    431712, 
    431713, 
    431714, 
    431715, 
    431716, 
    431717, 
    431718, 
    431719, 
    431720, 
    431721, 
    431722, 
    431723, 
    431724, 
    431725, 
    431726, 
    431727, 
    431728, 
    431729, 
    431730, 
    431731, 
    431732, 
    431733, 
    431743, 
    431744, 
    431745, 
    431746, 
    431747, 
    431748, 
    431749, 
    431750, 
    431751, 
    431752, 
    431753, 
    431754, 
    431755, 
    431756, 
    431757, 
    431758, 
    431759, 
    431760, 
    431761, 
    431762, 
    431763, 
    431764, 
    431765, 
    431766, 
    431776, 
    431777, 
    431778, 
    431779, 
    431780, 
    431781, 
    431782
  ) 
GROUP BY 
  p.product_id

Query time 0.00309

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 (396044,396045,396046,396047,396048,396049,396050,396052,396053,396054,396055,396056,396057,396074,396075,396076,396077,396078,396079,396080,396081,396082,396083,396084,396085,414329,414330,414331,414332,414333,414334,414335,414336,414337,414338,414339,414340,414341,414360,414361,414362,414363,414364,414365,414366,414393,414394,414395,414396,414397,414398,414399,414401,414402,414403,414404,414405,414406,414407,414415,414416,414417,414418,414419,414420,414421,414429,414430,414431,414432,414433,414434,414435,414459,414460,414461,414462,414463,414464,414465,414468,414470,414471,414472,414473,414474,414475,414476,414514,414516,414517,414518,414519,414520,414521,431514,431515,431516,431517,431518,431519,431520,431521,431522,431523,431524,431525,431526,431527,431528,431529,431530,431531,431532,431533,431534,431535,431536,431537,431687,431688,431689,431690,431691,431692,431693,431694,431695,431696,431697,431698,431699,431700,431701,431702,431703,431704,431705,431706,431707,431708,431709,431710,431711,431712,431713,431714,431715,431716,431717,431718,431719,431720,431721,431722,431723,431724,431725,431726,431727,431728,431729,431730,431731,431732,431733,431743,431744,431745,431746,431747,431748,431749,431750,431751,431752,431753,431754,431755,431756,431757,431758,431759,431760,431761,431762,431763,431764,431765,431766,431776,431777,431778,431779,431780,431781,431782)) 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"
            ]
          }
        }
      ]
    }
  }
}