exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/LimitExchangeTransposeRule.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.drill.exec.planner.physical;

import com.google.common.collect.ImmutableList;
import org.apache.calcite.plan.RelOptRule;
import org.apache.calcite.plan.RelOptRuleCall;
import org.apache.calcite.plan.hep.HepRelVertex;
import org.apache.calcite.plan.volcano.RelSubset;
import org.apache.calcite.rel.RelNode;
import org.apache.calcite.rex.RexLiteral;
import org.apache.calcite.rex.RexNode;
import org.apache.drill.exec.planner.logical.RelOptHelper;

import java.math.BigDecimal;

public class LimitExchangeTransposeRule extends Prule{
  public static final RelOptRule INSTANCE = new LimitExchangeTransposeRule();

  private boolean findRowKeyJoin(RelNode rel) {
    if (rel instanceof RowKeyJoinPrel) {
      return true;
    }
    if (rel instanceof RelSubset) {
      if (((RelSubset) rel).getBest() != null) {
        if (findRowKeyJoin(((RelSubset) rel).getBest())) {
          return true;
        }
      } else if (((RelSubset) rel).getOriginal() != null) {
        if (findRowKeyJoin(((RelSubset) rel).getOriginal())) {
          return true;
        }
      }
    } else if (rel instanceof HepRelVertex) {
      if (((HepRelVertex) rel).getCurrentRel() != null) {
        if (findRowKeyJoin(((HepRelVertex) rel).getCurrentRel())) {
          return true;
        }
      }
    } else {
      for (RelNode child : rel.getInputs()) {
        if (findRowKeyJoin(child)) {
          return true;
        }
      }
    }
    return false;
  }

  private LimitExchangeTransposeRule() {
    super(RelOptHelper.some(LimitPrel.class, RelOptHelper.any(ExchangePrel.class)), "LimitExchangeTransposeRule");
  }

  @Override
  public boolean matches(RelOptRuleCall call) {
    final LimitPrel limit = (LimitPrel) call.rel(0);
    final ExchangePrel exchange = (ExchangePrel) call.rel(1);

    //this rule now works for two exchanges only: UnionExchangePrel and SingleMergeExchangePrel
    if (!(exchange instanceof UnionExchangePrel) && !(exchange instanceof SingleMergeExchangePrel)) {
      return false;
    }

    // Two situations we do not fire this rule:
    // 1) limit has been pushed down to its child,
    // 2) the fetch() is null (indicating we have to fetch all the remaining rows starting from offset.
    if (!limit.isPushDown() && limit.getFetch() != null) {
      if (!(exchange instanceof SingleMergeExchangePrel)
          || !findRowKeyJoin(exchange)) {
        return true;
      }
    }
    return false;
  }

  @Override
  public void onMatch(RelOptRuleCall call) {
    final LimitPrel limit = (LimitPrel) call.rel(0);
    final ExchangePrel exchangePrel = (ExchangePrel) call.rel(1);

    RelNode child = exchangePrel.getInput();

    final int offset = limit.getOffset() != null ? Math.max(0, RexLiteral.intValue(limit.getOffset())) : 0;
    final int fetch = Math.max(0, RexLiteral.intValue(limit.getFetch()));

    // child Limit uses conservative approach:  use offset 0 and fetch = parent limit offset + parent limit fetch.
    final RexNode childFetch = limit.getCluster().getRexBuilder().makeExactLiteral(BigDecimal.valueOf(offset + fetch));

    final RelNode limitUnderExchange = new LimitPrel(child.getCluster(), child.getTraitSet(), child, null, childFetch);
    final RelNode newExch = exchangePrel.copy(exchangePrel.getTraitSet(), ImmutableList.of(limitUnderExchange));
    final RelNode limitAboveExchange = new LimitPrel(limit.getCluster(), limit.getTraitSet(), newExch, limit.getOffset(), limit.getFetch(), true);

    call.transformTo(limitAboveExchange);
  }

}