@Override
  public ParseContext transform(ParseContext pctx) throws SemanticException {
    // create a the context for walking operators
    FixedBucketPruningOptimizerCtxt opPartWalkerCtx = new FixedBucketPruningOptimizerCtxt(compat,
        pctx);

    // walk operator tree to create expression tree for filter buckets
    PrunerUtils.walkOperatorTree(pctx, opPartWalkerCtx,
        new BucketBitsetGenerator(), new NoopWalker());

    return pctx;
  }
Fixed bucket pruning optimizer goes through all the table scans and annotates them
with a bucketing inclusion bit-set.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/FixedBucketPruningOptimizer.java