code: func (t *tableCommon) WritableCols() []*table.Column {
  	if len(t.writableColumns) > 0 {
  		return t.writableColumns
  	}
  	writableColumns := make([]*table.Column, len(t.Columns))
  	maxOffset := -1
  	for _, col := range t.Columns {
  		if col.State == model.StateDeleteOnly || col.State == model.StateDeleteReorganization {
  			continue
  		}
  		writableColumns[col.Offset] = col
  		if maxOffset < col.Offset {
  			maxOffset = col.Offset
  		}
  	}
  	return writableColumns[0 : maxOffset+1]
  }
docs: // WritableCols implements table WritableCols interface.