public void setExprToColumnAlias(ASTNode expr, String alias) {
    exprToColumnAlias.put(expr,  StringInternUtils.internIfNotNull(alias));
  }
Get the Sort By AST for the clause.

@param clause
         the name of the clause
@return the abstract syntax tree
ql/src/java/org/apache/hadoop/hive/ql/parse/QBParseInfo.java