code: public ColumnValueSelector<?> makeColumnValueSelector(String columnName, ColumnSelectorFactory factory)
    {
      final VirtualColumn virtualColumn = getVirtualColumn(columnName);
      if (virtualColumn == null) {
        throw new IAE("No such virtual column[%s]", columnName);
      } else {
        final ColumnValueSelector<?> selector = virtualColumn.makeColumnValueSelector(columnName, factory);
        Preconditions.checkNotNull(selector, "selector");
        return selector;
      }
    }
docs: Create a column value selector.
  
  @param columnName column mame
  @param factory    base column selector factory
  
  @return selector
  
  @throws IllegalArgumentException if the virtual column does not exist (see {@link #exists(String)}