Github user yufeldman commented on a diff in the pull request:
https://github.com/apache/drill/pull/368#discussion_r61515592
--- Diff:
exec/java-exec/src/main/java/org/apache/drill/exec/planner/fragment/contrib/SimpleParallelizerMultiPlans.java
---
@@ -0,0 +1,228 @@
+/**
+ * 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.fragment.contrib;
+
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+
+import org.apache.drill.common.exceptions.ExecutionSetupException;
+import org.apache.drill.common.util.DrillStringUtils;
+import org.apache.drill.exec.ops.QueryContext;
+import org.apache.drill.exec.physical.base.Exchange;
+import org.apache.drill.exec.physical.base.FragmentRoot;
+import org.apache.drill.exec.physical.base.PhysicalOperator;
+import org.apache.drill.exec.planner.PhysicalPlanReader;
+import org.apache.drill.exec.planner.fragment.Fragment;
+import org.apache.drill.exec.planner.fragment.PlanningSet;
+import org.apache.drill.exec.planner.fragment.SimpleParallelizer;
+import org.apache.drill.exec.planner.fragment.Wrapper;
+import
org.apache.drill.exec.planner.fragment.Materializer.IndexedFragmentNode;
+import org.apache.drill.exec.proto.BitControl.PlanFragment;
+import org.apache.drill.exec.proto.BitControl.QueryContextInformation;
+import org.apache.drill.exec.proto.CoordinationProtos.DrillbitEndpoint;
+import org.apache.drill.exec.proto.ExecProtos.FragmentHandle;
+import org.apache.drill.exec.proto.UserBitShared.QueryId;
+import org.apache.drill.exec.rpc.user.UserSession;
+import org.apache.drill.exec.server.options.OptionList;
+import org.apache.drill.exec.work.QueryWorkUnit;
+import org.apache.drill.exec.work.foreman.ForemanSetupException;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.Lists;
+
+/**
+ * SimpleParallelizerMultiPlans class is an extension to SimpleParallelizer
+ * to help with getting PlanFragments for split plan.
+ * Split plan is essentially ability to create multiple Physical Operator
plans from original Physical Operator plan
+ * to be able to run plans separately.
+ * Moving functionality specific to splitting the plan to this class
+ * allows not to pollute parent class with non-authentic functionality
+ *
+ */
+public class SimpleParallelizerMultiPlans extends SimpleParallelizer {
+
+ static final org.slf4j.Logger logger =
org.slf4j.LoggerFactory.getLogger(SimpleParallelizerMultiPlans.class);
+
+ public SimpleParallelizerMultiPlans(QueryContext context) {
+ super(context);
+ }
+
+ /**
+ * Create multiple physical plans from original query planning, it will
allow execute them eventually independently
+ * @param options
+ * @param foremanNode
+ * @param queryId
+ * @param activeEndpoints
+ * @param reader
+ * @param rootFragment
+ * @param session
+ * @param queryContextInfo
+ * @return
+ * @throws ExecutionSetupException
+ */
+ public List<QueryWorkUnit> getSplitFragments(OptionList options,
DrillbitEndpoint foremanNode, QueryId queryId,
+ Collection<DrillbitEndpoint> activeEndpoints, PhysicalPlanReader
reader, Fragment rootFragment,
+ UserSession session, QueryContextInformation queryContextInfo)
throws ExecutionSetupException {
+
+ final PlanningSet planningSet = getFragmentsHelper(activeEndpoints,
rootFragment);
+
+ return generateWorkUnits(
+ options, foremanNode, queryId, reader, rootFragment, planningSet,
session, queryContextInfo);
+ }
+
+ /**
+ * Split plan into multiple plans based on parallelization
+ * Ideally it is applicable only to plans with two major fragments:
Screen and UnionExchange
+ * But there could be cases where we can remove even multiple exchanges
like in case of "order by"
--- End diff --
After discussing with Aman - in his own words:
Currently, Drill will generate the following type of plan for an ORDER BY:
SingleMergeExchange
|
Sort
|
HashToRandomExchange
|
Scan
This is for global ordering, but if you don't care about that, you can drop
the top SingleMergeExchange. The lower HashToRandomExchange is actually only
done to divide up the data in some way (we don't currently have RoundRobin
exchange), such that a single node does not end up doing all the sort.
However, if your data locality is such that data is somewhat uniformly
distributed already, you could even drop the lower exchange and do the local
Sort at each node or each minor fragment.
So, it sounds like you don't even care about global ordering, so it should
be ok to drop..
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