[638][df] Revert choice to change default Snapshot TTree compression settings

tree/dataframe/inc/ROOT/RDF/RInterface.hxx

@@ -1361,11 +1361,14 @@ public:
                return true;
          RLogScopedVerbosity showInfo{ROOT::Detail::RDF::RDFLogChannel(), ROOT::ELogLevel::kInfo};
          R__LOG_INFO(ROOT::Detail::RDF::RDFLogChannel())
-            << "\n\tIn ROOT 6.38, the default compression settings of Snapshot have been changed from 101 (ZLIB with "
-               "compression level 1, the TTree default) to 505 (ZSTD with compression level 5). This change may result "
-               "in smaller Snapshot output dataset size by default. In order to suppress this message, set "
-               "'ROOT_RDF_SNAPSHOT_INFO=0' in your environment or set 'ROOT.RDF.Snapshot.Info: 0' in your .rootrc "
-               "file.";
+            << "\n\tIn ROOT 6.38.00, the default compression settings of Snapshot were changed from 101 (ZLIB with "
+               "compression level 1, the TTree default) to 505 (ZSTD with compression level 5). The decision was based "
+               "on empirical evidence available up to that point. New studies summarised at "
+               "https://github.com/root-project/root/pull/21753 show that in certain cases "
+               "compression setting 101 is still the best option for TTree. Thus, this choice is reverted in ROOT "
+               "6.38.06 and later releases. "
+               "In order to suppress this message, set 'ROOT_RDF_SNAPSHOT_INFO=0' in your environment or set "
+               "'ROOT.RDF.Snapshot.Info: 0' in your .rootrc file.";
          return true;
       }();
       // like columnList but with `#var` columns removed

tree/dataframe/inc/ROOT/RSnapshotOptions.hxx

@@ -56,13 +56,13 @@ Note that for RNTuple, the defaults correspond to those set in RNTupleWriteOptio
 <td><code>fCompressionAlgorithm</code></td>
 <td><code>ROOT::RCompressionSetting::EAlgorithm</code></td>
 <td>Zstd</td>
-<td>Compression algorithm for the output dataset</td>
+<td>Compression algorithm for the output dataset, defaults to ROOT::RCompressionSetting::EAlgorithm::EValues::kUndefined. This is converted to ZLIB by default for TTree and ZSTD by default for RNTuple</td>
 </tr>
 <tr>
 <td><code>fCompressionLevel</code></td>
 <td><code>int</code></td>
 <td>5</td>
-<td>Compression level for the output dataset</td>
+<td>Compression level for the output dataset, defaults to 0 (uncompressed). If the default value of `fCompressionAlgorithm` is not modified, the compression level is changed to 1 by default for TTree and 5 by default for RNTuple</td>
 </tr>
 <tr>
 <td><code>fOutputFormat</code></td>
@@ -184,9 +184,8 @@ struct RSnapshotOptions {
    }
    std::string fMode = "RECREATE"; ///< Mode of creation of output file
    ESnapshotOutputFormat fOutputFormat = ESnapshotOutputFormat::kDefault; ///< Which data format to write to
-   ECAlgo fCompressionAlgorithm =
-      ROOT::RCompressionSetting::EAlgorithm::kZSTD; ///< Compression algorithm of output file
-   int fCompressionLevel = 5;                       ///< Compression level of output file
+   ECAlgo fCompressionAlgorithm = ECAlgo::kUndefined;                     ///< Compression algorithm of output file
+   int fCompressionLevel = 0;                                             ///< Compression level of output file
    bool fLazy = false;                              ///< Do not start the event loop when Snapshot is called
    bool fOverwriteIfExists = false;  ///< If fMode is "UPDATE", overwrite object in output file if it already exists
    bool fVector2RVec = true;         ///< If set to true will convert std::vector columns to RVec when saving to disk

tree/dataframe/src/RDFSnapshotHelpers.cxx

@@ -364,6 +364,28 @@ void SetBranchesHelper(TTree *inputTree, TTree &outputTree,
    throw std::logic_error(
       "RDataFrame::Snapshot: something went wrong when creating a TTree branch, please report this as a bug.");
 }
+
+auto GetSnapshotCompressionSettings(const ROOT::RDF::RSnapshotOptions &options)
+{
+   using CompAlgo = ROOT::RCompressionSetting::EAlgorithm::EValues;
+   using OutputFormat = ROOT::RDF::ESnapshotOutputFormat;
+
+   if (options.fOutputFormat == OutputFormat::kTTree || options.fOutputFormat == OutputFormat::kDefault) {
+      // The default compression settings for TTree is 101
+      if (options.fCompressionAlgorithm == CompAlgo::kUndefined) {
+         return ROOT::CompressionSettings(CompAlgo::kZLIB, 1);
+      }
+      return ROOT::CompressionSettings(options.fCompressionAlgorithm, options.fCompressionLevel);
+   } else if (options.fOutputFormat == OutputFormat::kRNTuple) {
+      // The default compression settings for RNTuple is 505
+      if (options.fCompressionAlgorithm == CompAlgo::kUndefined) {
+         return ROOT::CompressionSettings(CompAlgo::kZSTD, 5);
+      }
+      return ROOT::CompressionSettings(options.fCompressionAlgorithm, options.fCompressionLevel);
+   } else {
+      throw std::invalid_argument("RDataFrame::Snapshot: unrecognized output format");
+   }
+}
 } // namespace
 
 ROOT::Internal::RDF::RBranchData::RBranchData(std::string inputBranchName, std::string outputBranchName, bool isDefine,
@@ -535,8 +557,7 @@ void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::SetEmptyBranches(TTree *in
 void ROOT::Internal::RDF::UntypedSnapshotTTreeHelper::Initialize()
 {
    fOutputFile.reset(
-      TFile::Open(fFileName.c_str(), fOptions.fMode.c_str(), /*ftitle=*/"",
-                  ROOT::CompressionSettings(fOptions.fCompressionAlgorithm, fOptions.fCompressionLevel)));
+      TFile::Open(fFileName.c_str(), fOptions.fMode.c_str(), /*ftitle=*/"", GetSnapshotCompressionSettings(fOptions)));
    if (!fOutputFile)
       throw std::runtime_error("Snapshot: could not create output file " + fFileName);
 
@@ -774,9 +795,9 @@ void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::SetEmptyBranches(TTree *
 
 void ROOT::Internal::RDF::UntypedSnapshotTTreeHelperMT::Initialize()
 {
-   const auto cs = ROOT::CompressionSettings(fOptions.fCompressionAlgorithm, fOptions.fCompressionLevel);
    auto outFile =
-      std::unique_ptr<TFile>{TFile::Open(fFileName.c_str(), fOptions.fMode.c_str(), /*ftitle=*/fFileName.c_str(), cs)};
+      std::unique_ptr<TFile>{TFile::Open(fFileName.c_str(), fOptions.fMode.c_str(), /*ftitle=*/fFileName.c_str(),
+                                         GetSnapshotCompressionSettings(fOptions))};
    if (!outFile)
       throw std::runtime_error("Snapshot: could not create output file " + fFileName);
    fOutputFile = outFile.get();
@@ -918,7 +939,7 @@ void ROOT::Internal::RDF::UntypedSnapshotRNTupleHelper::Initialize()
    model->Freeze();
 
    ROOT::RNTupleWriteOptions writeOptions;
-   writeOptions.SetCompression(fOptions.fCompressionAlgorithm, fOptions.fCompressionLevel);
+   writeOptions.SetCompression(GetSnapshotCompressionSettings(fOptions));
    writeOptions.SetInitialUnzippedPageSize(fOptions.fInitialUnzippedPageSize);
    writeOptions.SetMaxUnzippedPageSize(fOptions.fMaxUnzippedPageSize);
    writeOptions.SetApproxZippedClusterSize(fOptions.fApproxZippedClusterSize);
@@ -1140,8 +1161,7 @@ ROOT::Internal::RDF::SnapshotHelperWithVariations::SnapshotHelperWithVariations(
 
    TDirectory::TContext fileCtxt;
    fOutputHandle = std::make_shared<SnapshotOutputWriter>(
-      TFile::Open(filename.data(), fOptions.fMode.c_str(), /*ftitle=*/"",
-                  ROOT::CompressionSettings(fOptions.fCompressionAlgorithm, fOptions.fCompressionLevel)));
+      TFile::Open(filename.data(), fOptions.fMode.c_str(), /*ftitle=*/"", GetSnapshotCompressionSettings(fOptions)));
    if (!fOutputHandle->fFile)
       throw std::runtime_error(std::string{"Snapshot: could not create output file "} + std::string{filename});
 

tree/dataframe/test/dataframe_snapshot.cxx

@@ -247,6 +247,22 @@ TEST(RDFSnapshotMore, BasketSizePreservation)
    TestBasketSizePreservation();
 }
 
+// Test for default compression settings
+TEST(RDFSnapshotMore, DefaultCompressionSettings)
+{
+   struct FileGuardRAII {
+      std::string fFilename{"RDFSnapshotMore_default_compression_settings.root"};
+      std::string fTreeName{"tree"};
+      ~FileGuardRAII() { std::remove(fFilename.c_str()); }
+   } fileGuard;
+   ROOT::RDataFrame df{1};
+   df.Define("x", [] { return 42; }).Snapshot(fileGuard.fTreeName, fileGuard.fFilename, {"x"});
+
+   auto f = std::make_unique<TFile>(fileGuard.fFilename.c_str());
+   EXPECT_EQ(f->GetCompressionAlgorithm(), ROOT::RCompressionSetting::EAlgorithm::EValues::kZLIB);
+   EXPECT_EQ(f->GetCompressionLevel(), 1);
+}
+
 // fixture that provides fixed and variable sized arrays as RDF columns
 class RDFSnapshotArrays : public ::testing::Test {
 protected:

tree/dataframe/test/dataframe_snapshot_ntuple.cxx

@@ -170,6 +170,26 @@ TEST(RDFSnapshotRNTuple, WriteOpts)
    }
 }
 
+TEST(RDFSnapshotRNTuple, DefaultCompressionSettings)
+{
+   FileRAII fileGuard{"RDFSnapshotRNTuple_default_compression_settings.root"};
+   const std::vector<std::string> columns = {"x"};
+
+   auto df = ROOT::RDataFrame(25ull).Define("x", [] { return 10; });
+
+   RSnapshotOptions opts;
+   opts.fOutputFormat = ROOT::RDF::ESnapshotOutputFormat::kRNTuple;
+
+   auto sdf = df.Snapshot("ntuple", fileGuard.GetPath(), {"x"}, opts);
+
+   EXPECT_EQ(columns, sdf->GetColumnNames());
+
+   auto reader = RNTupleReader::Open("ntuple", fileGuard.GetPath());
+   auto compSettings = *reader->GetDescriptor().GetClusterDescriptor(0).GetColumnRange(0).GetCompressionSettings();
+   // The RNTuple default should be 505
+   EXPECT_EQ(505, compSettings);
+}
+
 TEST(RDFSnapshotRNTuple, Compression)
 {
    FileRAII fileGuard{"RDFSnapshotRNTuple_compression.root"};