SELECT 
  promo_id, 
  discount, 
  coupon_code, 
  promo_to_date, 
  show_best_price_on, 
  short_description, 
  detailed_description 
FROM 
  kl_promo_vc_product_links pvpl 
  LEFT JOIN cscart_promotion_descriptions cpd ON pvpl.promo_id = cpd.promotion_id 
WHERE 
  pvpl.product_id = 31786 
  AND pvpl.is_discount_stale = 'N'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "4.20"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "pvpl",
          "access_type": "ref",
          "possible_keys": [
            "idx_product_id",
            "idx_is_discount_stale",
            "idx_product_is_discount_stale"
          ],
          "key": "idx_product_id",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "0.35",
            "data_read_per_join": "1K"
          },
          "used_columns": [
            "promo_id",
            "product_id",
            "discount",
            "coupon_code",
            "show_best_price_on",
            "is_discount_stale",
            "promo_to_date"
          ],
          "attached_condition": "(`cscart_db`.`pvpl`.`is_discount_stale` = 'N')"
        }
      },
      {
        "table": {
          "table_name": "cpd",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "promotion_id"
          ],
          "key_length": "3",
          "ref": [
            "cscart_db.pvpl.promo_id"
          ],
          "rows_examined_per_scan": 11,
          "rows_produced_per_join": 11,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "2.75",
            "eval_cost": "1.10",
            "prefix_cost": "4.20",
            "data_read_per_join": "26K"
          },
          "used_columns": [
            "promotion_id",
            "short_description",
            "detailed_description"
          ]
        }
      }
    ]
  }
}