private OpGroup newGroup(NodeProcessorCtx procCtx) {
      BucketVersionProcessorCtx ctx = (BucketVersionProcessorCtx) procCtx;
      OpGroup g = new OpGroup();
      ctx.groups.add(g);
      return g;
    }
This rule decomposes the operator tree into group which may have different bucketing versions.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/BucketVersionPopulator.java