private void evaluateMergeWork(MergeJoinWork w) throws SemanticException {
      for (BaseWork baseWork : w.getBaseWorkList()) {
        evaluateOperators(baseWork, pctx);
      }
    }
MemoryDecider is a simple physical optimizer that adjusts the memory layout of tez tasks.
Currently it only cares about hash table sizes for the graceful hash join.
It tried to keep hashtables that are small and early in the operator pipeline completely
in memory.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/physical/MemoryDecider.java