[GitHub] [arrow] pitrou commented on a change in pull request #8178: ARROW-9969: [C++] RecordBatch schema has correct types

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

pitrou commented on a change in pull request #8178:
URL: https://github.com/apache/arrow/pull/8178#discussion_r489250800



##########
File path: cpp/src/arrow/table_builder.cc
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@@ -62,7 +62,21 @@ Status RecordBatchBuilder::Flush(bool reset_builders,
     }
     length = fields[i]->length();
   }
-  *batch = RecordBatch::Make(schema_, length, std::move(fields));
+
+  // For certain types like dictionaries, types may not be fully
+  // determined before we have flushed. Make sure that the RecordBatch
+  // gets the correct types in schema.
+  // See: #ARROW-9969
+  std::shared_ptr<Schema> schema(schema_);
+  std::vector<std::shared_ptr<Field>> schema_fields(schema->fields());
+  for (int i = 0; i < this->num_fields(); ++i) {
+    if (!schema_fields[i]->type()->Equals(fields[i]->type())) {
+      std::shared_ptr<Field> new_type(schema_fields[i]->WithType(fields[i]->type()));
+      ARROW_ASSIGN_OR_RAISE(schema, schema->SetField(i, new_type));
+    }
+  }

Review comment:
       For the record, the entire loop may be O(N^2) if there are N types to replace.
   It may be better to do it in two steps:
   1. detect if at least one type needs replacing
   2. if so, create a new schema with a new fields vector
   
   Or you could always create a new schema anyway...




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