Github user bitblender commented on a diff in the pull request:

    https://github.com/apache/drill/pull/866#discussion_r131685349
  
    --- Diff: 
exec/java-exec/src/main/java/org/apache/drill/exec/physical/rowSet/impl/TupleSetImpl.java
 ---
    @@ -0,0 +1,551 @@
    +/*
    + * 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.rowSet.impl;
    +
    +import java.util.ArrayList;
    +import java.util.List;
    +
    +import org.apache.drill.common.types.TypeProtos.DataMode;
    +import org.apache.drill.exec.expr.TypeHelper;
    +import org.apache.drill.exec.physical.rowSet.ColumnLoader;
    +import org.apache.drill.exec.physical.rowSet.TupleLoader;
    +import org.apache.drill.exec.physical.rowSet.TupleSchema;
    +import 
org.apache.drill.exec.physical.rowSet.impl.ResultSetLoaderImpl.VectorContainerBuilder;
    +import org.apache.drill.exec.record.BatchSchema;
    +import org.apache.drill.exec.record.BatchSchema.SelectionVectorMode;
    +import org.apache.drill.exec.record.MaterializedField;
    +import org.apache.drill.exec.vector.AllocationHelper;
    +import org.apache.drill.exec.vector.ValueVector;
    +import org.apache.drill.exec.vector.VectorOverflowException;
    +import org.apache.drill.exec.vector.accessor.impl.AbstractColumnWriter;
    +import org.apache.drill.exec.vector.accessor.impl.ColumnAccessorFactory;
    +
    +/**
    + * Implementation of a column when creating a row batch.
    + * Every column resides at an index, is defined by a schema,
    + * is backed by a value vector, and and is written to by a writer.
    + * Each column also tracks the schema version in which it was added
    + * to detect schema evolution. Each column has an optional overflow
    + * vector that holds overflow record values when a batch becomes
    + * full.
    + * <p>
    + * Overflow vectors require special consideration. The vector class itself
    + * must remain constant as it is bound to the writer. To handle overflow,
    + * the implementation must replace the buffer in the vector with a new
    + * one, saving the full vector to return as part of the final row batch.
    + * This puts the column in one of three states:
    + * <ul>
    + * <li>Normal: only one vector is of concern - the vector for the active
    + * row batch.</li>
    + * <li>Overflow: a write to a vector caused overflow. For all columns,
    + * the data buffer is shifted to a harvested vector, and a new, empty
    + * buffer is put into the active vector.</li>
    + * <li>Excess: a (small) column received values for the row that will
    --- End diff --
    
    'Excess' is the LOOK_AHEAD state, correct? I think it would be better if 
the comments use the same terminology as in the code.


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