code: func renewColumns(oldCol []*column, cap int) []*column {
  	columns := make([]*column, 0, len(oldCol))
  	for _, col := range oldCol {
  		if col.isFixed() {
  			columns = append(columns, newFixedLenColumn(len(col.elemBuf), cap))
  		} else {
  			columns = append(columns, newVarLenColumn(cap, col))
  		}
  	}
  	return columns
  }
docs: // renewColumns creates the columns of a Chunk. The capacity of the newly
  // created columns is equal to cap.