Add Stats to Tablet Constants and Renaming (facebookincubator#74)

Summary:
Pull Request resolved: facebookincubator#74

Raw size will be implemented in a followup task, to better reflect logical data size that is file format agnostic.

# Changes
- Adding the optional Stats section to Tablet

- ~~`VeloxWriter.cpp`: Calculating the raw data size in bytes via running sum of memory used right before encoding and flushing~~
  - ~~`FieldWriter.h`: Added `rawSize()` virtual function to handle string and nullable cases.~~

# Important Notes

- In `VeloxWriter.cpp` we have special handling for chunked null streams, which is why `materialize` is called in an if statement.

Differential Revision: D60534808

dwio/nimble/encodings/RleEncoding.h

@@ -174,7 +174,7 @@ class RLEEncodingBase
 
 } // namespace internal
 
-// Handles the numeric cases. Bools and strings are templated below.
+// Handles the numeric and string cases. Bools are templated below.
 // Data layout is:
 // RLEEncodingBase bytes
 // 4 * sizeof(physicalType) bytes: run values

dwio/nimble/tablet/Constants.h

@@ -31,5 +31,6 @@ constexpr uint32_t kPostscriptChecksumedSize = 5;
 
 constexpr std::string_view kSchemaSection = "columnar.schema";
 constexpr std::string_view kMetadataSection = "columnar.metadata";
+constexpr std::string_view kStatsSection = "columnar.stats";
 
 } // namespace facebook::nimble

dwio/nimble/velox/VeloxWriter.cpp

@@ -15,14 +15,11 @@
  */
 #include "dwio/nimble/velox/VeloxWriter.h"
 
-#include <ios>
 #include <memory>
 
 #include "dwio/nimble/common/Exceptions.h"
 #include "dwio/nimble/common/Types.h"
-#include "dwio/nimble/encodings/Encoding.h"
 #include "dwio/nimble/encodings/EncodingSelectionPolicy.h"
-#include "dwio/nimble/encodings/SentinelEncoding.h"
 #include "dwio/nimble/tablet/Constants.h"
 #include "dwio/nimble/velox/BufferGrowthPolicy.h"
 #include "dwio/nimble/velox/ChunkedStreamWriter.h"
@@ -557,6 +554,8 @@ void VeloxWriter::close() {
              builder.GetSize()});
       }
 
+      // TODO: logical raw size.
+
       {
         SchemaSerializer serializer;
         writer_.writeOptionalSection(
@@ -621,14 +620,12 @@ void VeloxWriter::writeChunk(bool lastChunk) {
     streams_.resize(context_->schemaBuilder.nodeCount());
 
     // When writing null streams, we write the nulls as data, and the stream
-    // itself is non-nullable. This adpater class is how we expose the nulls as
-    // values.
+    // itself is non-nullable. This adapter class is how we expose the nulls
+    // as values.
     class NullsAsDataStreamData : public StreamData {
      public:
       explicit NullsAsDataStreamData(StreamData& streamData)
-          : StreamData(streamData.descriptor()), streamData_{streamData} {
-        streamData_.materialize();
-      }
+          : StreamData(streamData.descriptor()), streamData_{streamData} {}
 
       inline virtual std::string_view data() const override {
         return {
@@ -692,6 +689,16 @@ void VeloxWriter::writeChunk(bool lastChunk) {
              streamData.nonNulls().size() > minStreamSize) ||
             (lastChunk && !streamData.empty() &&
              !streams_[offset].content.empty())) {
+          // If we ended up in this code block, it means that we either saw
+          // enough nulls to flush a chunk, or that we are on the last chunk of
+          // a stream containing nulls.
+          // If previous chunks in the same stream contained nulls, then
+          // regardless of the presence of nulls in the last chunk we must flush
+          // it. If the last chunk doesn't contain nulls, the DataStream (which
+          // is a NullsStreamData) will contain an empty nulls buffer, so we
+          // must materialize it before we write.
+          streamData.materialize();
+
           encoder(streamData, true);
         }
       } else {