SELECT 
  feature_id, 
  feature_type 
FROM 
  cscart_product_features 
WHERE 
  feature_id IN (18, 549, 45, 1137, 548, 15, 16, 17)

Query time 0.00043

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "19.21"
    },
    "table": {
      "table_name": "cscart_product_features",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "feature_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 8,
      "rows_produced_per_join": 8,
      "filtered": "100.00",
      "index_condition": "(`danishecarter_25_march`.`cscart_product_features`.`feature_id` in (18,549,45,1137,548,15,16,17))",
      "cost_info": {
        "read_cost": "17.61",
        "eval_cost": "1.60",
        "prefix_cost": "19.21",
        "data_read_per_join": "9K"
      },
      "used_columns": [
        "feature_id",
        "feature_type"
      ]
    }
  }
}