rdblue commented on code in PR #461: URL: https://github.com/apache/parquet-format/pull/461#discussion_r1859087543
########## VariantShredding.md: ########## @@ -25,290 +25,316 @@ The 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 process is **shredding**. -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 upon the [Variant Binary Encoding](VariantEncoding.md), and leverages the existing Parquet specification. +Shredding enables the use of Parquet's columnar representation for more compact data encoding, column statistics for data skipping, and partial projections. -At a high level, we replace the `value` field of the Variant Parquet group with one or more fields called `object`, `array`, `typed_value`, and `variant_value`. -These represent a fixed schema suitable for constructing the full Variant value for each row. +For example, the query `SELECT variant_get(event, '$.event_ts', 'timestamp') FROM tbl` only needs to load field `event_ts`, and if that column is shredded, it can be read by columnar projection without reading or deserializing the rest of the `event` Variant. +Similarly, for the query `SELECT * FROM tbl WHERE variant_get(event, '$.event_type', 'string') = 'signup'`, the `event_type` shredded column metadata can be used for skipping and to lazily load the rest of the Variant. -Shredding allows a query engine to 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). -Without shredding, any query that accesses a Variant column must fetch all bytes of the full binary buffer. -With shredding, we can get nearly equivalent performance as in a relational (scalar) data model. +## Variant Metadata -For example, `select variant_get(variant_col, ‘$.field1.inner_field2’, ‘string’) from tbl` only needs to access `inner_field2`, and the file scan could avoid fetching the rest of the Variant value if this field was shredded into a separate column in the Parquet schema. -Similarly, for the query `select * from tbl where variant_get(variant_col, ‘$.id’, ‘integer’) = 123`, the scan could first decode the shredded `id` column, and only fetch/decode the full Variant value for rows that pass the filter. +Variant metadata is stored in the top-level Variant group in a binary `metadata` column regardless of whether the Variant value is shredded. -# Parquet Example +All `value` columns within the Variant must use the same `metadata`. +All field names of a Variant, whether shredded or not, must be present in the metadata. -Consider the following Parquet schema together with how Variant values might be mapped to it. -Notice that we represent each shredded field in `object` as a group of two fields, `typed_value` and `variant_value`. -We extract all homogenous data items of a certain path into `typed_value`, and set aside incompatible data items in `variant_value`. -Intuitively, incompatibilities within the same path may occur because we store the shredding schema per Parquet file, and each file can contain several row groups. -Selecting a type for each field that is acceptable for all rows would be impractical because it would require buffering the contents of an entire file before writing. +## Value Shredding -Typically, the expectation is that `variant_value` exists at every level as an option, along with one of `object`, `array` or `typed_value`. -If the actual Variant value contains a type that does not match the provided schema, it is stored in `variant_value`. -An `variant_value` may also be populated if an object can be partially represented: any fields that are present in the schema must be written to those fields, and any missing fields are written to `variant_value`. - -The `metadata` column is unchanged from its unshredded representation, and may be referenced in `variant_value` fields in the shredded data. +Variant values are stored in Parquet fields named `value`. +Each `value` field may have an associated shredded field named `typed_value` that stores the value when it matches a specific type. Review Comment: Isn't this clear? It says "an associated shredded field" with a specific name. I'm not sure how that could be interpreted to allow multiple fields in this version of the spec. -- 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: issues-unsubscr...@parquet.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@parquet.apache.org For additional commands, e-mail: issues-h...@parquet.apache.org
