private boolean putInternal(String tabAlias, String colAlias, ColumnInfo colInfo) {
    if (!addMappingOnly(tabAlias, colAlias, colInfo)) {
      //Make sure that the table alias and column alias are stored
      //in the column info
      if (tabAlias != null) {
        colInfo.setTabAlias(tabAlias.toLowerCase());
      }
      rowSchema.getSignature().add(colInfo);
      return true;
    }
    return false;
  }
Retrieves the source expression matching a given ASTNode's
string rendering exactly.
ql/src/java/org/apache/hadoop/hive/ql/parse/RowResolver.java