private List<FieldNode> getWindowFunctionColumns(WindowTableFunctionDef tDef) {
      List<FieldNode> columns = new ArrayList<>();
      if (tDef.getWindowFunctions() != null) {
        for (WindowFunctionDef wDef : tDef.getWindowFunctions()) {
          columns.add(new FieldNode(wDef.getAlias()));
        }
      }
      return columns;
    }
- Pruning can only be done for Windowing. PTFs are black boxes,
  we assume all columns are needed.
- add column names referenced in WindowFn args and in WindowFn expressions
  to the pruned list of the child Select Op.
- finally we set the prunedColList on the ColumnPrunerContx;
  and update the RR &amp; signature on the PTFOp.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ColumnPrunerProcFactory.java