[GitHub] [arrow] bkietz commented on a change in pull request #7608: ARROW-9288: [C++][Dataset] Fix PartitioningFactory with dictionary encoding for HivePartioning

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

bkietz commented on a change in pull request #7608:
URL: https://github.com/apache/arrow/pull/7608#discussion_r451690654



##########
File path: cpp/src/arrow/dataset/partition.cc
##########
@@ -646,15 +657,26 @@ class HivePartitioningFactory : public PartitioningFactory {
       }
     }
 
-    return impl.Finish(&dictionaries_);
+    auto schema_result = impl.Finish(&dictionaries_);
+    field_names_ = impl.FieldNames();
+    return schema_result;

Review comment:
       Finish doesn't mutate `name_to_index_` and that is the only data member accessed by `FieldNames()`. I don't see why Finish needs to be called first
   ```suggestion
       field_names_ = impl.FieldNames();
       return impl.Finish(&dictionaries_);
   ```

##########
File path: cpp/src/arrow/dataset/partition.cc
##########
@@ -646,15 +657,26 @@ class HivePartitioningFactory : public PartitioningFactory {
       }
     }
 
-    return impl.Finish(&dictionaries_);
+    auto schema_result = impl.Finish(&dictionaries_);
+    field_names_ = impl.FieldNames();
+    return schema_result;
   }
 
   Result<std::shared_ptr<Partitioning>> Finish(
       const std::shared_ptr<Schema>& schema) const override {
-    return std::shared_ptr<Partitioning>(new HivePartitioning(schema, dictionaries_));
+    for (FieldRef ref : field_names_) {
+      // ensure all of field_names_ are present in schema
+      RETURN_NOT_OK(ref.FindOne(*schema).status());
+    }
+
+    // drop fields which aren't in field_names_
+    auto out_schema = SchemaFromColumnNames(schema, field_names_);
+
+    return std::make_shared<HivePartitioning>(std::move(out_schema), dictionaries_);

Review comment:
       The check against field_names_ is only relevant if dictionaries_ is non-empty, which can only occur if Inspect has been called (and `field_names_` has therefore been initialized)
   ```suggestion
       if (dictionaries_.empty()) {
         return std::make_shared<HivePartitioning>(schema, dictionaries_);
       } else {
         for (FieldRef ref : field_names_) {
           // ensure all of field_names_ are present in schema
           RETURN_NOT_OK(ref.FindOne(*schema).status());
         }
   
         // drop fields which aren't in field_names_
         auto out_schema = SchemaFromColumnNames(schema, field_names_);
   
         return std::make_shared<HivePartitioning>(std::move(out_schema), dictionaries_);
       }
   ```




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