code: func rangePartitionString(pi *model.PartitionInfo) string {
  	// partition by range expr
  	if len(pi.Columns) == 0 {
  		return pi.Expr
  	}
  
  	// partition by range columns (c1)
  	if len(pi.Columns) == 1 {
  		return pi.Columns[0].L
  	}
  
  	// partition by range columns (c1, c2, ...)
  	panic("create table assert len(columns) = 1")
  }
docs: // rangePartitionString returns the partition string for a range typed partition.