Github user paul-rogers commented on a diff in the pull request:

    https://github.com/apache/drill/pull/1121#discussion_r169203884
  
    --- Diff: 
exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/protocol/VectorContainerAccessor.java
 ---
    @@ -0,0 +1,132 @@
    +/*
    + * 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.physical.impl.protocol;
    +
    +import java.util.Collections;
    +import java.util.Iterator;
    +
    +import org.apache.drill.common.expression.SchemaPath;
    +import org.apache.drill.exec.record.BatchSchema;
    +import org.apache.drill.exec.record.TypedFieldId;
    +import org.apache.drill.exec.record.VectorContainer;
    +import org.apache.drill.exec.record.VectorWrapper;
    +import org.apache.drill.exec.record.WritableBatch;
    +import org.apache.drill.exec.record.selection.SelectionVector2;
    +import org.apache.drill.exec.record.selection.SelectionVector4;
    +
    +public class VectorContainerAccessor implements BatchAccessor {
    +
    +  public static class ContainerAndSv2Accessor extends 
VectorContainerAccessor {
    +
    +    private SelectionVector2 sv2;
    +
    +    public void setSelectionVector(SelectionVector2 sv2) {
    +      this.sv2 = sv2;
    +    }
    +
    +    @Override
    +    public SelectionVector2 getSelectionVector2() {
    +      return sv2;
    +    }
    +  }
    +
    +  public static class ContainerAndSv4Accessor extends 
VectorContainerAccessor {
    +
    +    private SelectionVector4 sv4;
    +
    +    @Override
    +    public SelectionVector4 getSelectionVector4() {
    +      return sv4;
    +    }
    +  }
    +
    +  private VectorContainer container;
    +  private SchemaTracker schemaTracker = new SchemaTracker();
    +
    +  /**
    +   * Set the vector container. Done initially, and any time the schema of
    +   * the container may have changed. May be called with the same container
    +   * as the previous call, or a different one. A schema change occurs
    +   * unless the vectors are identical across the two containers.
    +   *
    +   * @param container the container that holds vectors to be sent
    +   * downstream
    +   */
    +
    +  public void setContainer(VectorContainer container) {
    +    this.container = container;
    +    if (container != null) {
    +      schemaTracker.trackSchema(container);
    +    }
    +  }
    +
    +  @Override
    +  public BatchSchema getSchema() {
    +    return container == null ? null : container.getSchema();
    +  }
    +
    +  @Override
    +  public int schemaVersion() { return schemaTracker.schemaVersion(); }
    +
    +  @Override
    +  public int getRowCount() {
    +    return container == null ? 0 : container.getRecordCount();
    +  }
    +
    +  @Override
    +  public VectorContainer getOutgoingContainer() { return container; }
    +
    +  @Override
    +  public TypedFieldId getValueVectorId(SchemaPath path) {
    +    return container.getValueVectorId(path);
    +  }
    +
    +  @Override
    +  public VectorWrapper<?> getValueAccessorById(Class<?> clazz, int... ids) 
{
    +    return container.getValueAccessorById(clazz, ids);
    +  }
    +
    +  @Override
    +  public WritableBatch getWritableBatch() {
    +    return WritableBatch.get(container);
    +  }
    +
    +  @Override
    +  public SelectionVector2 getSelectionVector2() {
    +    // Throws an exception by default
    --- End diff --
    
    We could. The reason that it throws an exception is that, if the batch has 
no selection vector, yet we ask for it, it is an error under Drill semantics. 
Said another way, the client should ask for a selection vector only if one is 
available. This is existing behavior; this method simply wraps that existing 
behavior.
    
    So, the place to document the exception is on the `VectorContainer` method.
    
    Improved the comment a bit.


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