private Path createDummyFile() throws SemanticException {
    Path dummyPath = new Path(ctx.getMRScratchDir(), "dummy_path");
    Path dummyFile = new Path(dummyPath, "dummy_file");
    FSDataOutputStream fout = null;
    try {
      FileSystem fs = dummyFile.getFileSystem(conf);
      if (fs.exists(dummyFile)) {
        return dummyPath;
      }
      fout = fs.create(dummyFile);
      fout.write(1);
      fout.close();
    } catch (IOException e) {
      throw new SemanticException(e);
    } finally {
      IOUtils.closeStream(fout);
    }
    return dummyPath;
  }
if the column is a skewed column, use ColumnInfo accordingly
ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java