code: func builtinCurrentTime(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	fsp := 0
  	sc := ctx.GetSessionVars().StmtCtx
  	if len(args) == 1 && !args[0].IsNull() {
  		if fsp, err = checkFsp(sc, args[0]); err != nil {
  			d.SetNull()
  			return d, errors.Trace(err)
  		}
  	}
  	d.SetString(time.Now().Format("15:04:05.000000"))
  	return convertToDuration(ctx.GetSessionVars().StmtCtx, d, fsp)
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_curtime