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 (
    275133, 275759, 275765, 276261, 276352, 
    276420, 276421, 276426, 276427, 276428, 
    276429, 276430, 276431, 276432, 276433, 
    276434, 276435, 276436, 276437, 276438, 
    276439, 276440, 276441, 276442, 276443, 
    276444, 276445, 276446, 276447, 276448, 
    276449, 276450, 276451, 276452, 276453, 
    276454, 276455, 276456, 276457, 276458, 
    276459, 276460, 276461, 276462, 276463, 
    276464, 276465, 276466, 276467, 276468, 
    276469, 276470, 276471, 276472, 276473, 
    276474, 276475, 276476, 276477, 276478, 
    276497, 276498, 276499, 276500, 276501, 
    276502, 276503, 276504, 276505, 276506, 
    276507, 276508, 276509, 276510, 276511, 
    276512, 276513, 276514, 276515, 276516
  ) 
GROUP BY 
  p.product_id

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.70"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 8,
            "rows_produced_per_join": 1,
            "filtered": "12.50",
            "cost_info": {
              "read_cost": "1.22",
              "eval_cost": "0.10",
              "prefix_cost": "1.32",
              "data_read_per_join": "448"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`s2cart`.`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": [
              "s2cart.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.01",
              "prefix_cost": "1.67",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`s2cart`.`cscart_discussion`.`object_id` in (275133,275759,275765,276261,276352,276420,276421,276426,276427,276428,276429,276430,276431,276432,276433,276434,276435,276436,276437,276438,276439,276440,276441,276442,276443,276444,276445,276446,276447,276448,276449,276450,276451,276452,276453,276454,276455,276456,276457,276458,276459,276460,276461,276462,276463,276464,276465,276466,276467,276468,276469,276470,276471,276472,276473,276474,276475,276476,276477,276478,276497,276498,276499,276500,276501,276502,276503,276504,276505,276506,276507,276508,276509,276510,276511,276512,276513,276514,276515,276516)) and (`s2cart`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "s2cart.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.02",
              "eval_cost": "0.01",
              "prefix_cost": "1.70",
              "data_read_per_join": "393"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}