SELECT 
  `D`.`name`, 
  `D`.`discount_id`, 
  `D`.`time_to`, 
  `D`.`path` 
FROM 
  `cscart_discount_companies` AS `DC` 
  LEFT JOIN `cscart_discounts` AS `D` ON `D`.`discount_id` = `DC`.`discount_id` 
WHERE 
  `DC`.`company_id` = 687

Query time 0.00053

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.65"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "DC",
          "access_type": "ALL",
          "rows_examined_per_scan": 3,
          "rows_produced_per_join": 1,
          "filtered": "33.33",
          "cost_info": {
            "read_cost": "0.45",
            "eval_cost": "0.10",
            "prefix_cost": "0.55",
            "data_read_per_join": "16"
          },
          "used_columns": [
            "company_id",
            "discount_id"
          ],
          "attached_condition": "(`dev2`.`DC`.`company_id` = 687)"
        }
      },
      {
        "table": {
          "table_name": "D",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "discount_id"
          ],
          "key_length": "4",
          "ref": [
            "dev2.DC.discount_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "1.00",
            "eval_cost": "0.10",
            "prefix_cost": "1.65",
            "data_read_per_join": "2K"
          },
          "used_columns": [
            "discount_id",
            "name",
            "time_to",
            "path"
          ]
        }
      }
    ]
  }
}