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 (
    8007, 7724, 7897, 7952, 7959, 7972, 7974, 
    8009, 8010, 7948, 7949, 7953, 7962, 
    7964, 7968, 8005, 8013, 7231, 7249, 
    7391, 7947, 7955, 7957, 7958, 7975, 
    7992, 7024, 7355, 7893, 7954, 7970, 
    7977, 7196, 7350, 8080, 7227, 7324, 
    7500, 7956, 8006, 7022, 7080, 7191, 
    7250, 7320, 7340, 7352, 7888, 8110, 
    8192, 8364, 7021, 7192, 7256, 7506, 
    7850, 7230, 7319, 7343, 7386, 7900, 
    8214, 7189, 7226, 7348, 7356, 7390, 
    7965, 7232, 7237, 7344, 7383, 7389, 
    7887, 7894, 8207, 9909, 7193, 7262, 
    8069
  ) 
GROUP BY 
  p.product_id

Query time 0.00100

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 (8007,7724,7897,7952,7959,7972,7974,8009,8010,7948,7949,7953,7962,7964,7968,8005,8013,7231,7249,7391,7947,7955,7957,7958,7975,7992,7024,7355,7893,7954,7970,7977,7196,7350,8080,7227,7324,7500,7956,8006,7022,7080,7191,7250,7320,7340,7352,7888,8110,8192,8364,7021,7192,7256,7506,7850,7230,7319,7343,7386,7900,8214,7189,7226,7348,7356,7390,7965,7232,7237,7344,7383,7389,7887,7894,8207,9909,7193,7262,8069)) 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"
            ]
          }
        }
      ]
    }
  }
}