[SPARK-9517][SQL] BytesToBytesMap should encode data the same way as UnsafeExternalSorter

BytesToBytesMap current encodes key/value data in the following format:
```
8B key length, key data, 8B value length, value data
```

UnsafeExternalSorter, on the other hand, encodes data this way:
```
4B record length, data
```

As a result, we cannot pass records encoded by BytesToBytesMap directly into UnsafeExternalSorter for sorting. However, if we rearrange data slightly, we can then pass the key/value records directly into UnsafeExternalSorter:
```
4B key+value length, 4B key length, key data, value data
```

Author: Reynold Xin <rxin@databricks.com>

Closes #7845 from rxin/kvsort-rebase and squashes the following commits:

5716b59 [Reynold Xin] Fixed test.
2e62ccb [Reynold Xin] Updated BytesToBytesMap's data encoding to put the key first.
a51b641 [Reynold Xin] Added a KV sorter interface.

1 files changed, 30 insertions, 0 deletions

diff --git a/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeKeyValueSorter.java b/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeKeyValueSorter.java
new file mode 100644
index 0000000000..59c774da74
--- /dev/null
+++ b/sql/catalyst/src/main/java/org/apache/spark/sql/execution/UnsafeKeyValueSorter.java
@@ -0,0 +1,30 @@
+/*
+ * 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;
+
+import java.io.IOException;
+
+import org.apache.spark.sql.catalyst.expressions.UnsafeRow;
+import org.apache.spark.unsafe.KVIterator;
+
+public abstract class UnsafeKeyValueSorter {
+
+  public abstract void insert(UnsafeRow key, UnsafeRow value);
+
+  public abstract KVIterator<UnsafeRow, UnsafeRow> sort() throws IOException;
+}