Github user chenghao-intel commented on a diff in the pull request:

    https://github.com/apache/spark/pull/1880#discussion_r16039955
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/columnar/InMemoryColumnarTableScan.scala
 ---
    @@ -90,22 +101,31 @@ private[sql] case class InMemoryColumnarTableScan(
     
       override def execute() = {
         relation.cachedColumnBuffers.mapPartitions { iterator =>
    -      val columnBuffers = iterator.next()
    -      assert(!iterator.hasNext)
    +      // Find the ordinals of the requested columns.  If none are 
requested, use the first.
    +      val requestedColumns =
    +        if (attributes.isEmpty) {
    +          Seq(0)
    +        } else {
    +          attributes.map(a => relation.output.indexWhere(_.exprId == 
a.exprId))
    +        }
     
           new Iterator[Row] {
    -        // Find the ordinals of the requested columns.  If none are 
requested, use the first.
    -        val requestedColumns =
    -          if (attributes.isEmpty) {
    -            Seq(0)
    -          } else {
    -            attributes.map(a => relation.output.indexWhere(_.exprId == 
a.exprId))
    -          }
    +        private[this] var columnBuffers: Array[ByteBuffer] = null
    +        private[this] var columnAccessors: Seq[ColumnAccessor] = null
    +        nextBatch()
    --- End diff --
    
    @marmbrus @liancheng It's great to see we will support the smaller batch 
processing in `InMemoryRelation`, but Ideally, we can rewind the bytebuffers in 
the beginning of each iteration in `InMemoryRelation` during the  runtime, the 
whole detail should be transparent to the iterator in 
`InMemoryColumnarTableScan`. In this way, I think we can keep most of the code 
unchanged in `InMemoryColumnarTableScan`  then.


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