[SPARK-9611] [SQL] Fixes a few corner cases when we spill a UnsafeFixedWidthAggregationMap

This PR has the following three small fixes.

1. UnsafeKVExternalSorter does not use 0 as the initialSize to create an UnsafeInMemorySorter if its BytesToBytesMap is empty.
2. We will not not spill a InMemorySorter if it is empty.
3. We will not add a SpillReader to a SpillMerger if this SpillReader is empty.

JIRA: https://issues.apache.org/jira/browse/SPARK-9611

Author: Yin Huai <yhuai@databricks.com>

Closes #7948 from yhuai/unsafeEmptyMap and squashes the following commits:

9727abe [Yin Huai] Address Josh's comments.
34b6f76 [Yin Huai] 1. UnsafeKVExternalSorter does not use 0 as the initialSize to create an UnsafeInMemorySorter if its BytesToBytesMap is empty. 2. Do not spill a InMemorySorter if it is empty. 3. Do not add spill to SpillMerger if this spill is empty.

1 files changed, 4 insertions, 2 deletions

diff --git a/sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java b/sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java
index 86a563df99..6c1cf136d9 100644
--- a/sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java
+++ b/sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java
@@ -82,8 +82,11 @@ public final class UnsafeKVExternalSorter {
         pageSizeBytes);
     } else {
       // Insert the records into the in-memory sorter.
+      // We will use the number of elements in the map as the initialSize of the
+      // UnsafeInMemorySorter. Because UnsafeInMemorySorter does not accept 0 as the initialSize,
+      // we will use 1 as its initial size if the map is empty.
       final UnsafeInMemorySorter inMemSorter = new UnsafeInMemorySorter(
-        taskMemoryManager, recordComparator, prefixComparator, map.numElements());
+        taskMemoryManager, recordComparator, prefixComparator, Math.max(1, map.numElements()));
 
       final int numKeyFields = keySchema.size();
       BytesToBytesMap.BytesToBytesMapIterator iter = map.iterator();
@@ -214,7 +217,6 @@ public final class UnsafeKVExternalSorter {
           // Note that recordLen = keyLen + valueLen + 4 bytes (for the keyLen itself)
           int keyLen = PlatformDependent.UNSAFE.getInt(baseObj, recordOffset);
           int valueLen = recordLen - keyLen - 4;
-
           key.pointTo(baseObj, recordOffset + 4, numKeyFields, keyLen);
           value.pointTo(baseObj, recordOffset + 4 + keyLen, numValueFields, valueLen);