public void initColumn(int colIx, int streamCount) {
    hasData[colIx] = true;
    if (columnData[colIx] == null || columnData[colIx].length != streamCount) {
      columnData[colIx] = new ColumnStreamData[streamCount];
    }
  }
Indicates which columns have data. This is indexed by the column ID in ORC file schema;
the indices that are not included will not have data. Correspond to columnData elements.
storage-api/src/java/org/apache/hadoop/hive/common/io/encoded/EncodedColumnBatch.java