Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/7904#discussion_r36580644
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/joins/RowIterator.scala
---
@@ -0,0 +1,63 @@
+/*
+ * 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.spark.sql.execution.joins
+
+import java.util.NoSuchElementException
+
+import org.apache.spark.sql.catalyst.InternalRow
+
+private[sql] abstract class RowIterator {
+ def advanceNext(): Boolean
+ def getRow: InternalRow
+ def toScala: Iterator[InternalRow] = new RowIteratorToScala(this)
+}
+
+object RowIterator {
+ def fromScala(scalaIter: Iterator[InternalRow]): RowIterator = {
+ scalaIter match {
+ case wrappedRowIter: RowIteratorToScala => wrappedRowIter.rowIter
+ case _ => new RowIteratorFromScala(scalaIter)
+ }
+ }
+}
+
+private final class RowIteratorToScala(val rowIter: RowIterator) extends
Iterator[InternalRow] {
+ private [this] var _hasNext: Boolean = rowIter.advanceNext()
+ override def hasNext: Boolean = _hasNext
+ override def next(): InternalRow = {
+ if (!_hasNext) throw new NoSuchElementException
+ val row: InternalRow = rowIter.getRow.copy()
--- End diff --
I was a little inprecise in describing the problem: what if, while
processing the current row, we want to see whether there is another row, so we
call `hasNext()` but do _not_ want to do the work of mutating the current value
from the iterator (e.g. we want the mutable value to only be updated by next
calls, not `hasNext()`) and we also want to have an idempotent `hasNext()`?
In principle, I suppose that `HashJoin` is vulnerable to problem: what if
the row returned by the iterator points to the last value for a particular left
row and we call `hasNext()` without intending to immediately call `next()`? In
that case, I think that we'll wind up advancing the row as a result of the
`hasNext()` call.
Maybe this concern is moot. I was just trying to figure out why the copy
was necessary here and this was the only explanation that I could come up with.
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