SELECT 
  D.variant 
FROM 
  cscart_product_feature_variant_descriptions AS D 
  LEFT JOIN cscart_product_feature_variants AS V ON D.variant_id = V.variant_id 
WHERE 
  D.link = '285' 
  AND V.feature_id = 737

Query time 0.00126

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "152.46"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "V",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "feature_id"
          ],
          "key": "feature_id",
          "used_key_parts": [
            "feature_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 338,
          "rows_produced_per_join": 338,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.31",
            "eval_cost": "33.80",
            "prefix_cost": "34.11",
            "data_read_per_join": "512K"
          },
          "used_columns": [
            "variant_id",
            "feature_id"
          ],
          "attached_condition": "(`dev2`.`V`.`variant_id` is not null)"
        }
      },
      {
        "table": {
          "table_name": "D",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id"
          ],
          "key_length": "3",
          "ref": [
            "dev2.V.variant_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 33,
          "filtered": "10.00",
          "cost_info": {
            "read_cost": "84.50",
            "eval_cost": "3.38",
            "prefix_cost": "152.46",
            "data_read_per_join": "143K"
          },
          "used_columns": [
            "variant_id",
            "variant",
            "link"
          ],
          "attached_condition": "(`dev2`.`D`.`link` = '285')"
        }
      }
    ]
  }
}