SELECT 
  id, 
  vendor_id, 
  store 
FROM 
  ushop_wk_buynow 
WHERE 
  vendor_id = 15

Query time 0.00025

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.00"
    },
    "table": {
      "table_name": "ushop_wk_buynow",
      "access_type": "system",
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "0.00",
        "eval_cost": "0.10",
        "prefix_cost": "0.00",
        "data_read_per_join": "24"
      },
      "used_columns": [
        "id",
        "vendor_id",
        "store"
      ]
    }
  }
}