Parquet summary files are not particular useful nowadays since
- when schema merging is disabled, we assume schema of all Parquet part-files are identical, thus we can read the footer from any part-files.
- when schema merging is enabled, we need to read footers of all files anyway to do the merge.
On the other hand, writing summary files can be expensive because footers of all part-files must be read and merged. This is particularly costly when appending small dataset to large existing Parquet dataset.