private void keepAmbiguousInfo(String col_alias, String tab_alias) {
    // we keep track of duplicate <tab alias, col alias> so that get can check
    // for ambiguity
    Map<String, String> colAliases = ambiguousColumns.get(tab_alias);
    if (colAliases == null) {
      colAliases = new LinkedHashMap<String, String>();
      ambiguousColumns.put(tab_alias, colAliases);
    }
    colAliases.put(col_alias, col_alias );
  }
Puts a resolver entry corresponding to the expression and its
column alias which is used for alias recognition in the latter constructs
@param exprToColumnAlias
         The map containing the expression to alias mapping
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/parse/RowResolver.java