SELECT 
  feature_id 
FROM 
  cscart_product_features_values 
WHERE 
  variant_id = 6403

Query time 0.00121

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "121.10"
    },
    "table": {
      "table_name": "cscart_product_features_values",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "variant_id",
        "idx_product_feature_variant_id"
      ],
      "key": "variant_id",
      "used_key_parts": [
        "variant_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 346,
      "rows_produced_per_join": 346,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "86.50",
        "eval_cost": "34.60",
        "prefix_cost": "121.10",
        "data_read_per_join": "267K"
      },
      "used_columns": [
        "feature_id",
        "variant_id"
      ]
    }
  }
}