[GitHub] [arrow] bkietz commented on a change in pull request #8301: ARROW-10100: [C++][Python][Dataset] Add ParquetFileFragment::Subset method

[GitBox <gi...@apache.org>]

bkietz commented on a change in pull request #8301:
URL: https://github.com/apache/arrow/pull/8301#discussion_r502461050



##########
File path: cpp/src/arrow/dataset/file_parquet.cc
##########
@@ -631,6 +631,39 @@ Result<FragmentVector> ParquetFileFragment::SplitByRowGroup(
   return fragments;
 }
 
+Result<std::shared_ptr<Fragment>> ParquetFileFragment::Subset(
+    const std::shared_ptr<Expression>& predicate) {
+  RETURN_NOT_OK(EnsureCompleteMetadata());
+  ARROW_ASSIGN_OR_RAISE(auto row_groups, FilterRowGroups(*predicate));
+
+  ARROW_ASSIGN_OR_RAISE(auto new_fragment,
+                        parquet_format_.MakeFragment(source_, partition_expression(),
+                                                     row_groups, physical_schema_));
+  return new_fragment;

Review comment:
       ```suggestion
     return checked_pointer_cast<ParquetFileFragment>(std::move(new_fragment));
   ```

##########
File path: cpp/src/arrow/dataset/file_parquet.h
##########
@@ -232,6 +232,10 @@ class ARROW_DS_EXPORT ParquetFileFragment : public FileFragment {
   /// \brief Ensure attached statistics are complete and the physical schema is cached.
   Status EnsureCompleteMetadata(parquet::arrow::FileReader* reader = NULLPTR);
 
+  /// \brief Return a filtered subset of the ParquetFileFragment.
+  Result<std::shared_ptr<Fragment>> Subset(const std::shared_ptr<Expression>& predicate);
+  Result<std::shared_ptr<Fragment>> Subset(const std::vector<int> row_group_ids);

Review comment:
       ```suggestion
     Result<std::shared_ptr<ParquetFileFragment>> Subset(const std::shared_ptr<Expression>& predicate);
     Result<std::shared_ptr<ParquetFileFragment>> Subset(const std::vector<int> row_group_ids);
   ```




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