@Override
    protected void generatePredicate(NodeProcessorCtx procCtx, FilterOperator fop,
                                     TableScanOperator top) throws SemanticException, UDFArgumentException {
      OpWalkerCtx owc = (OpWalkerCtx) procCtx;
      // Otherwise this is not a sampling predicate and we need to
      ExprNodeDesc predicate = fop.getConf().getPredicate();
      String alias = top.getConf().getAlias();

      // Generate the partition pruning predicate
      ExprNodeDesc ppr_pred = ExprProcFactory.genPruner(alias, predicate);

      // Add the pruning predicate to the table scan operator
      addPruningPred(owc.getOpToPartPruner(), top, ppr_pred);
    }
Determines the partition pruner for the filter. This is called only when
the filter follows a table scan operator.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ppr/OpProcFactory.java