diff --git a/velox/common/file/File.h b/velox/common/file/File.h
index 603ed28ba20e..32f8540eb3ac 100644
--- a/velox/common/file/File.h
+++ b/velox/common/file/File.h
@@ -235,16 +235,27 @@ struct HeapMemoryMock {
   explicit HeapMemoryMock(void* memory, size_t capacity)
       : memory_(memory), capacity_(capacity) {}
 
+  void reset() {
+    memory_ = nullptr;
+    size_ = 0;
+    capacity_ = 0;
+  }
+
   bool isValid() const {
     return memory_ != nullptr;
   }
 
-  void write(const void* data, size_t len) {
+  void write(const void* src, size_t len) {
     assert(len <= freeSize());
-    memcpy(end(), data, len);
+    memcpy(end(), src, len);
     size_ += len;
   }
 
+  void read(void* dst, size_ len, size_t offset) {
+    assert(offset + len <= size_);
+    memcpy(dst, (char*)memory_ + offset, len);
+  }
+
   auto size() const {
     return size_;
   }
@@ -268,28 +279,29 @@ struct HeapMemoryMock {
 
 const size_t kHeapMemoryCapacity = 64 * 1024;
 
-class HeapMemoryManagerMock {
+class HeapMemoryMockManager {
  public:
-  static HeapMemoryManagerMock& instance() {
-    static HeapMemoryManagerMock hmmm;
+  static HeapMemoryMockManager& instance() {
+    static HeapMemoryMockManager hmmm;
     return hmmm;
   }
 
   HeapMemoryMock alloc(size_t size) {
-    HeapMemoryMock handle;
+    HeapMemoryMock heapMemory;
     if (size_ + size <= kHeapMemoryCapacity) {
-      handle.memory_ = malloc(size);
-      handle.size_ = 0;
-      handle.capacity_ = size;
+      heapMemory.memory_ = malloc(size);
+      heapMemory.size_ = 0;
+      heapMemory.capacity_ = size;
       size_ += size;
     }
-    return handle;
+    return heapMemory;
   }
 
-  void free(HeapMemoryMock handle) {
-    if (handle.isValid()) {
-      size_ -= handle.size_;
-      ::free(handle.memory_);
+  void free(HeapMemoryMock& heapMemory) {
+    if (heapMemory.isValid()) {
+      size_ -= heapMemory.size_;
+      ::free(heapMemory.memory_);
+      heapMemory.reset();
     }
   }
 
@@ -298,23 +310,69 @@ class HeapMemoryManagerMock {
 };
 
 inline HeapMemoryMock allocHeapMemory(size_t size) {
-  return HeapMemoryManagerMock::instance().alloc(size);
+  return HeapMemoryMockManager::instance().alloc(size);
 }
 
-inline void freeHeapMemory(HeapMemoryMock handle) {
-  HeapMemoryManagerMock::instance().free(handle);
+inline void freeHeapMemory(HeapMemoryMock& heapMemory) {
+  HeapMemoryMockManager::instance().free(heapMemory);
 }
 
-class HeapMemoryWriteFile final : public WriteFile {
+class HeapMemoryReadFile : public ReadFile {
  public:
-  explicit HeapMemoryWriteFile(HeapMemoryMock handle) : handle_(handle) {}
+  explicit HeapMemoryReadFile(HeapMemoryMock& heapMemory)
+      : heapMemory_(heapMemory) {}
+
+  std::string_view pread(
+      uint64_t offset,
+      uint64_t length,
+      void* FOLLY_NONNULL buf) const override {
+    bytesRead_ += length;
+    heapMemory_.read(buf, length, offset);
+    return {static_cast<char*>(buf), length};
+  }
 
-  ~HeapMemoryWriteFile() {
-    freeHeapMemory(handle_);
+  std::string pread(uint64_t offset, uint64_t length) const override {
+    bytesRead_ += length;
+    assert(offset + lenght <= heapMemory_.size());
+    return std::string((char*)heapMemory_.begin() + offset, length);
   }
 
+  uint64_t size() const final {
+    return heapMemory_.size();
+  }
+
+  uint64_t memoryUsage() const final {
+    return size();
+  }
+
+  // Mainly for testing. Coalescing isn't helpful for in memory data.
+  void setShouldCoalesce(bool shouldCoalesce) {
+    shouldCoalesce_ = shouldCoalesce;
+  }
+  bool shouldCoalesce() const final {
+    return shouldCoalesce_;
+  }
+
+  std::string getName() const override {
+    return "<HeapMemoryReadFile>";
+  }
+
+  uint64_t getNaturalReadSize() const override {
+    return 1024;
+  }
+
+ private:
+  HeapMemoryMock& heapMemory_;
+  bool shouldCoalesce_ = false;
+};
+
+class HeapMemoryWriteFile final : public WriteFile {
+ public:
+  explicit HeapMemoryWriteFile(HeapMemoryMock& heapMemory)
+      : heapMemory_(heapMemory) {}
+
   void append(std::string_view data) final {
-    handle_.write(data.data(), data.length());
+    heapMemory_.write(data.data(), data.length());
   }
 
   void flush() final {}
@@ -322,11 +380,11 @@ class HeapMemoryWriteFile final : public WriteFile {
   void close() final {}
 
   uint64_t size() const final {
-    return handle_.size_;
+    return heapMemory_.size_;
   }
 
  private:
-  HeapMemoryMock handle_;
+  HeapMemoryMock& heapMemory_;
 };
 
 // Current implementation for the local version is quite simple (e.g. no
diff --git a/velox/exec/Spill.cpp b/velox/exec/Spill.cpp
index 0e6a02057e61..926cac340cf3 100644
--- a/velox/exec/Spill.cpp
+++ b/velox/exec/Spill.cpp
@@ -46,12 +46,14 @@ void SpillMergeStream::pop() {
 }
 
 void SpillFile::newOutput() {
-  auto handle = allocHeapMemory(targetFileSize_);
-  if (handle.isValid()) {
-    output_ = std::make_unique<HeapMemoryWriteFile>(handle);
+  heapMemoryMock_ = allocHeapMemory(targetFileSize_);
+  if (heapMemoryMock_.isValid()) {
+    output_ = std::make_unique<HeapMemoryWriteFile>(heapMemoryMock_);
+    toWhere_ = TO_HEAP;
   } else {
     auto fs = filesystems::getFileSystem(path_, nullptr);
     output_ = fs->openFileForWrite(path_);
+    toWhere_ = TO_FILE;
   }
 }
 
@@ -65,13 +67,23 @@ WriteFile& SpillFile::output() {
 void SpillFile::startRead() {
   constexpr uint64_t kMaxReadBufferSize =
       (1 << 20) - AlignedBuffer::kPaddedSize; // 1MB - padding.
+
   VELOX_CHECK(!output_);
   VELOX_CHECK(!input_);
-  auto fs = filesystems::getFileSystem(path_, nullptr);
-  auto file = fs->openFileForRead(path_);
-  auto buffer = AlignedBuffer::allocate<char>(
-      std::min<uint64_t>(fileSize_, kMaxReadBufferSize), &pool_);
-  input_ = std::make_unique<SpillInput>(std::move(file), std::move(buffer));
+
+  if (toWhere_ == TO_FILE) {
+    auto fs = filesystems::getFileSystem(path_, nullptr);
+    auto file = fs->openFileForRead(path_);
+    auto buffer = AlignedBuffer::allocate<char>(
+        std::min<uint64_t>(fileSize_, kMaxReadBufferSize), &pool_);
+    input_ = std::make_unique<SpillInput>(std::move(file), std::move(buffer));
+  } else {
+    std::unique_ptr<ReadFile> =
+        std::make_unique<HeapMemoryReadFile>(heapMemoryMock_);
+    auto buffer = AlignedBuffer::allocate<char>(
+        std::min<uint64_t>(fileSize_, kMaxReadBufferSize), &pool_);
+    input_ = std::make_unique<SpillInput>(std::move(file), std::move(buffer));
+  }
 }
 
 bool SpillFile::nextBatch(RowVectorPtr& rowVector) {
diff --git a/velox/exec/Spill.h b/velox/exec/Spill.h
index eabe302cfd8c..fad81116d32b 100644
--- a/velox/exec/Spill.h
+++ b/velox/exec/Spill.h
@@ -84,6 +84,12 @@ class SpillFile {
         sortCompareFlags_.size() == numSortingKeys_);
   }
 
+  ~SpillFile() {
+    if (heapMemoryMock_.isValid()) {
+      freeHeapMemory(heapMemoryMock_);
+    }
+  }
+
   int32_t numSortingKeys() const {
     return numSortingKeys_;
   }
@@ -149,6 +155,13 @@ class SpillFile {
   const int32_t ordinal_;
   const std::string path_;
 
+  enum {
+    TO_FILE,
+    TO_HEAP,
+  } toWhere_ = TO_FILE;
+
+  HeapMemoryMock heapMemoryMock_;
+
   // Byte size of the backing file. Set when finishing writing.
   uint64_t fileSize_ = 0;
   uint64_t targetFileSize_ = 0;