RussellSpitzer commented on code in PR #46831: URL: https://github.com/apache/spark/pull/46831#discussion_r1683562693
########## common/variant/shredding.md: ########## @@ -0,0 +1,244 @@ +# Shredding Overview + +The Spark Variant type is designed to store and process semi-structured data efficiently, even with heterogeneous values. Query engines encode each variant value in a self-describing format, and store it as a group containing **value** and **metadata** binary fields in Parquet. Since data is often partially homogenous, it can be beneficial to extract certain fields into separate Parquet columns to further improve performance. We refer to this process as "shredding". Each Parquet file remains fully self-describing, with no additional metadata required to read or fully reconstruct the Variant data from the file. Combining shredding with a binary residual provides the flexibility to represent complex, evolving data with an unbounded number of unique fields while limiting the size of file schemas, and retaining the performance benefits of a columnar format. + +This document focuses on the shredding semantics, Parquet representation, implications for readers and writers, as well as the Variant reconstruction. For now, it does not discuss which fields to shred, user-facing API changes, or any engine-specific considerations like how to use shredded columns. The approach builds on top of the generic Spark Variant representation, and leverages the existing Parquet specification for maximum compatibility with the open-source ecosystem. + +At a high level, we replace the **value** and **metadata** of the Variant Parquet group with one or more fields called **object**, **array**, **typed_value** and **untyped_value**. These represent a fixed schema suitable for constructing the full Variant value for each row. + +Shredding lets Spark (or any other query engine) reap the full benefits of Parquet's columnar representation, such as more compact data encoding, min/max statistics for data skipping, and I/O and CPU savings from pruning unnecessary fields not accessed by a query (including the non-shredded Variant binary data). Review Comment: nit: and IO and CPU savings If this was in the comma separated section earlier that would fit better -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: reviews-unsubscr...@spark.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: reviews-unsubscr...@spark.apache.org For additional commands, e-mail: reviews-h...@spark.apache.org
