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 (
    2187, 2244, 2172, 2171, 1668, 8586, 8372, 
    2222, 2175, 2228, 2220, 2177, 2230, 
    2226, 1667, 2221, 2179, 1666, 8240, 
    2183, 8813, 2176, 8433, 2224, 2195, 
    2225, 8514, 2189, 2196, 2186, 2180, 
    2178, 2191, 2170, 2181, 2190, 2185, 
    2198, 2182, 2188
  ) 
GROUP BY 
  p.product_id

Query time 0.00187

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.31"
    },
    "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": 13,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.70",
              "eval_cost": "0.13",
              "prefix_cost": "1.83",
              "data_read_per_join": "582"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.33",
              "eval_cost": "0.01",
              "prefix_cost": "2.28",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_id` in (2187,2244,2172,2171,1668,8586,8372,2222,2175,2228,2220,2177,2230,2226,1667,2221,2179,1666,8240,2183,8813,2176,8433,2224,2195,2225,8514,2189,2196,2186,2180,2178,2191,2170,2181,2190,2185,2198,2182,2188)) and (`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.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": "2.31",
              "data_read_per_join": "294"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}