@Override
  public ParseContext transform(ParseContext pactx) throws SemanticException {
    Set<String> bigTables = getBigTables(pactx);
    cache.clear();

    for (JoinOperator joinOp : pactx.getJoinOps()) {
      reorder(joinOp, bigTables);
    }

    return pactx;
  }
Transform the query tree. For each join, check which reduce sink will
output the biggest result (based on STREAMTABLE hints) and give it the
biggest tag so that it gets streamed.

@param pactx
         current parse context
ql/src/java/org/apache/hadoop/hive/ql/optimizer/JoinReorder.java