org.apache.hadoop.hive.ql.optimizer.physical.SortMergeJoinResolver Maven / Gradle / Ivy
/**
* 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.hadoop.hive.ql.optimizer.physical;
import java.util.ArrayList;
import java.util.List;
import org.apache.hadoop.hive.ql.lib.Dispatcher;
import org.apache.hadoop.hive.ql.lib.Node;
import org.apache.hadoop.hive.ql.lib.TaskGraphWalker;
import org.apache.hadoop.hive.ql.parse.SemanticException;
/*
* If a join has been automatically converted into a sort-merge join, create a conditional
* task to try map-side join with each table as the big table. It is similar to
* hive.auto.convert.join, but is only applicable to joins which have been automatically
* converted to sort-merge joins. For hive.auto.convert.join, the backup task is the
* map-reduce join, whereas here, the backup task is the sort-merge join.
*
* Depending on the inputs, a sort-merge join may be faster or slower than the map-side join.
* The other advantage of sort-merge join is that the output is also bucketed and sorted.
* Consider a very big table, say 1TB with 10 buckets being joined with a very small table, say
* 10MB with 10 buckets, the sort-merge join may perform slower since it will be restricted to
* 10 mappers.
*/
public class SortMergeJoinResolver implements PhysicalPlanResolver {
@Override
public PhysicalContext resolve(PhysicalContext pctx) throws SemanticException {
// create dispatcher and graph walker
Dispatcher disp = new SortMergeJoinTaskDispatcher(pctx);
TaskGraphWalker ogw = new TaskGraphWalker(disp);
// get all the tasks nodes from root task
List topNodes = new ArrayList();
topNodes.addAll(pctx.rootTasks);
// begin to walk through the task tree.
ogw.startWalking(topNodes, null);
return pctx;
}
}