tajo-core/src/main/java/org/apache/tajo/engine/planner/physical/HashLeftSemiJoinExec.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.tajo.engine.planner.physical;

import org.apache.tajo.plan.logical.JoinNode;
import org.apache.tajo.storage.Tuple;
import org.apache.tajo.worker.TaskAttemptContext;

import java.io.IOException;
import java.util.List;

/**
 * Prepare a hash table of the NOT IN side of the join. Scan the FROM side table.
 * For each tuple of the FROM side table, it tries to find a matched tuple from the hash table for the NOT INT side.
 * If found, it returns the tuple of the FROM side table.
 */
public class HashLeftSemiJoinExec extends HashJoinExec {

  private final List<Tuple> nullTupleList = nullTupleList(0);

  public HashLeftSemiJoinExec(TaskAttemptContext context, JoinNode plan, PhysicalExec fromSideChild,
                              PhysicalExec inSideChild) {
    super(context, plan, fromSideChild, inSideChild);
  }

  /**
   * The End of Tuple (EOT) condition is true only when no more tuple in the left relation (on disk).
   * next() method finds the first unmatched tuple from both tables.
   *
   * For each left tuple on the disk, next() tries to find at least one matched tuple from the hash table.
   *
   * In more detail, until there is a hash bucket matched to the left tuple in the hash table, it continues to traverse
   * the left tuples. If next() finds the matched bucket in the hash table, it finds any matched tuple in the bucket.
   * If found, it returns the composite tuple immediately without finding more matched tuple in the bucket.
   *
   * @return The tuple which is firstly matched to a given join condition.
   * @throws java.io.IOException
   */
  @Override
  public Tuple next() throws IOException {
    if (first) {
      loadRightToHashTable();
    }

    while(!context.isStopped() && !finished) {
      if (iterator != null && iterator.hasNext()) {
        frameTuple.setRight(iterator.next());
        return projector.eval(frameTuple);
      }
      // getting new outer
      Tuple leftTuple = leftChild.next(); // it comes from a disk
      if (leftTuple == null || leftFiltered(leftTuple)) { // if no more tuples in left tuples on disk, a join is completed.
        finished = leftTuple == null;
        continue;
      }

      frameTuple.setLeft(leftTuple);

      // Try to find a hash bucket in in-memory hash table
      TupleList hashed = tupleSlots.get(leftKeyExtractor.project(leftTuple));
      if (hashed != null && rightFiltered(hashed).hasNext()) {
        // if found, it gets a hash bucket from the hash table.
        iterator = nullTupleList.iterator();
      }
    }
    return null;
  }
}