[gdubya]

The technique can be applied by any engine, not just DataFusion. Each engine would have to know about the indexes in order to make use of them, but the fallback to parquet standard defaults means that the data is still readable by all.

[aerzen]

But does data fusion publish a specification of how this metadata can be read, along with a test suite for verifying implementations? Because if they don't, this cannot be reliably used by any other impl

[jasim]

Parquet files include a field called key_value_metadata in the FileMetadata structure; it sits in the footer of the file. See: https://github.com/apache/parquet-format/blob/master/src/mai...

The technique described in the article, seems to use this key-value pair to store pointers to the additional metadata (in this case a distinct index) embedded in the file. Note that we can embed arbitrary binary data in the Parquet file between each data page. This is perfectly valid since all Parquet readers rely on the exact offsets to the data pages specified in the footer.

This means that DataFusion does not need to specify how the metadata is interpreted. It is already well specified as part of the Parquet file format itself. DataFusion is an independent project -- it is a query execution engine for OLAP / columnar data, which can take in SQL statements, build query plan, optimize them, and execute. It is an embeddable runtime with numerous ways to extend it by the host program. Parquet is a file format supported by DataFusion because it is one of the most popular ways of storing data in a columnar way in object storages like S3.

Note that the readers of Parquet need to be aware of any metadata to exploit it. But if not, nothing changes - as long as we're embedding only supplementary information like indices or bloom filters, a reader can still continue working with the columnar data in Parquet as it used to; it is just that it won't be able to take advantage of the additional metadata.

[DAlperin]

The story here isn't that they've invented a new format for user defined indexes (the one proposed here is sort of contrived and I probably wouldn't recommend in production) but rather demonstrating how the user defined metadata space of the parquet format can be used for application specific purposes.

I work on a database engine that uses parquet as our on-storage file format and we make liberal use of the custom metadata area for things specific to our product that any other parquet readers would just ignore.