rcfile/src/main/java/com/twitter/elephantbird/mapreduce/output/RCFileOutputFormat.java
public static void setColumnNumber(Configuration conf, int columnNum) {
    assert columnNum > 0;
    conf.setInt(RCFile.COLUMN_NUMBER_CONF_STR, columnNum);
  }
public static void setColumnNumber(Configuration conf, int columnNum) {
    assert columnNum > 0;
    conf.setInt(RCFile.COLUMN_NUMBER_CONF_STR, columnNum);
  }
set number of columns into the given configuration.

@param conf
configuration instance which need to set the column number
@param columnNum
column number for RCFile's Writer