SELECT 
  variant_descriptions.variant 
FROM 
  q_product_features_values AS v 
  LEFT JOIN q_product_feature_variant_descriptions AS variant_descriptions ON variant_descriptions.variant_id = v.variant_id 
WHERE 
  v.product_id = 30188 
  AND v.feature_id = 2

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.60"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "v",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "product_id",
            "fpl"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "feature_id",
            "product_id"
          ],
          "key_length": "6",
          "ref": [
            "const",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "1.00",
            "eval_cost": "0.20",
            "prefix_cost": "1.20",
            "data_read_per_join": "792"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "variant_descriptions",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "variant_id"
          ],
          "key_length": "3",
          "ref": [
            "portal.v.variant_id"
          ],
          "rows_examined_per_scan": 2,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "2.00",
            "eval_cost": "0.40",
            "prefix_cost": "3.60",
            "data_read_per_join": "6K"
          },
          "used_columns": [
            "variant_id",
            "variant"
          ]
        }
      }
    ]
  }
}