ASTNode getAST() {
    return this.ast;
  }
DFS-scan the expressionTree to find all aggregation subtrees and put them
in aggregations.

@param expressionTree
@param aggregations
         the key to the HashTable is the toStringTree() representation of
         the aggregation subtree.
@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java