[df] Change default Snapshot compression settings

The default Snapshot compression setting has always been 101. Historically, this was done for simplicity reasons and following the principle of least surprise. TTree was the only output format available with Snapshot, so the operation was defaulting to the same value used by TTree. Now that Snapshot supports more than one output format, this reason is less strong than before. It has been established that 505 is a better default compression setting overall, so RDataFrame should follow that. The main disadvantage is that this change is hard to communicate. This commit proposes to introduce an information message being shown once per program execution, and only if the program is calling Snapshot. This message is supposed to help the users detect changes in their output file size with the next development cycle (6.38.x) and should be removed afterwards.

Author: vepadulano

README/ReleaseNotes/v638/index.md

@@ -116,6 +116,7 @@ If you want to keep using `TList*` return values, you can write a small adapter
   to numbers such as 8 would share one 3-d histogram among 8 threads, greatly reducing the memory consumption. This might slow down execution if the histograms
   are filled at very high rates. Use lower number in this case.
 - The Snapshot method has been refactored so that it does not need anymore compile-time information (i.e. either template arguments or JIT-ting) to know the input column types. This means that any Snapshot call that specifies the template arguments, e.g. `Snapshot<int, float>(..., {"intCol", "floatCol"})` is now redundant and the template arguments can safely be removed from the call. At the same time, Snapshot does not need to JIT compile the column types, practically giving huge speedups depending on the number of columns that need to be written to disk. In certain cases (e.g. when writing O(10000) columns) the speedup can be larger than an order of magnitude. The Snapshot template is now deprecated and it will issue a compile-time warning when called. The function overload is scheduled for removal in ROOT 6.40.
+- The default compression setting for the output dataset used by Snapshot has been changed from 101 (ZLIB level 1) to 505 (ZSTD level 5). This is a better setting on average, and makes more sense for RDataFrame since now the Snapshot operation supports more than just the TTree output data format. This change may result in smaller output file sizes for your analyses that use Snapshot with default settings.
 
 ## Python Interface
 

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

@@ -32,6 +32,7 @@
 #include "ROOT/RResultPtr.hxx"
 #include "ROOT/RSnapshotOptions.hxx"
 #include <string_view>
+#include "ROOT/RLogger.hxx"
 #include "ROOT/RVec.hxx"
 #include "ROOT/TypeTraits.hxx"
 #include "RtypesCore.h" // for ULong64_t
@@ -60,6 +61,7 @@
 #include <utility> // std::index_sequence
 #include <vector>
 #include <any>
+#include <mutex>
 
 class TGraph;
 
@@ -1331,6 +1333,13 @@ public:
                                                  const ColumnNames_t &columnList,
                                                  const RSnapshotOptions &options = RSnapshotOptions())
    {
+      // TODO: Remove before releasing 6.40.00
+      static std::once_flag once;
+      std::call_once(once, []() {
+         R__LOG_INFO(ROOT::Detail::RDF::RDFLogChannel())
+            << "The default compression settings of Snapshot has been changed from 101 (ZLIB with compression level 1) "
+               "to 505 (ZSTD with compression level 5).";
+      });
       // like columnList but with `#var` columns removed
       auto colListNoPoundSizes = RDFInternal::FilterArraySizeColNames(columnList, "Snapshot");
       // like columnListWithoutSizeColumns but with aliases resolved

tree/dataframe/inc/ROOT/RSnapshotOptions.hxx

@@ -46,8 +46,8 @@ struct RSnapshotOptions {
    }
    std::string fMode = "RECREATE"; ///< Mode of creation of output file
    ECAlgo fCompressionAlgorithm =
-      ROOT::RCompressionSetting::EAlgorithm::kZLIB; ///< Compression algorithm of output file
-   int fCompressionLevel = 1;                       ///< Compression level of output file
+      ROOT::RCompressionSetting::EAlgorithm::kZSTD; ///< Compression algorithm of output file
+   int fCompressionLevel = 5;                       ///< Compression level of output file
    int fAutoFlush = 0;                              ///< AutoFlush value for output tree
    int fSplitLevel = 99;                            ///< Split level of output tree
    bool fLazy = false;                              ///< Do not start the event loop when Snapshot is called