public void putExpression(ASTNode node, ColumnInfo colInfo) {
    String treeAsString = node.toStringTree();
    expressionMap.put(treeAsString, node);
    if (!putInternal("", treeAsString, colInfo)) {
      return;
    }
    colInfo.setAlias(treeAsString);
  }
Puts a resolver entry corresponding to a source expression which is to be
used for identical expression recognition (e.g. for matching expressions
in the SELECT list with the GROUP BY clause).  The convention for such
entries is an empty-string ("") as the table alias together with the
string rendering of the ASTNode as the column alias.
ql/src/java/org/apache/hadoop/hive/ql/parse/RowResolver.java