@Override
  public String toString() {
    StringBuilder sb = new StringBuilder();
    sb.append("Primary Key for " + databaseName+"."+tableName+":");
    sb.append("[");
    if (colNames != null && colNames.size() > 0) {
      for (Map.Entry<Integer, String> me : colNames.entrySet()) {
        sb.append(me.getValue()+",");
      }
      sb.setLength(sb.length()-1);
    }
    sb.append("], Constraint Name: " + constraintName);
    return sb.toString();
  }
PrimaryKeyInfo is a metadata structure containing the primary key associated with a table.
The fields include the table name, database name, constraint name, 
mapping of the position of the primary key column to the column name.
The position is one-based index.
ql/src/java/org/apache/hadoop/hive/ql/metadata/PrimaryKeyInfo.java