sfc-gh-aixu commented on code in PR #461: URL: https://github.com/apache/parquet-format/pull/461#discussion_r1809161104
########## VariantShredding.md: ########## @@ -33,140 +33,247 @@ This document focuses on the shredding semantics, Parquet representation, implic 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. -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. - 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. +With shredding, readers can get nearly equivalent performance as in a relational (scalar) data model. + +For example, `SELECT variant_get(variant_event, '$.event_ts', 'timestamp') FROM tbl` only needs to access `event_ts`, and the file scan could avoid fetching the rest of the Variant value if this field was shredded into a separate column in the Parquet schema. +Similarly, for the query `SELECT * FROM tbl WHERE variant_get(variant_event, '$.event_type', 'string') = 'signup'`, the scan could first decode the shredded `event_type` column, and only fetch/decode the full Variant event value for rows that pass the filter. -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 -# Parquet Example +Variant metadata is stored in the top-level Variant group in a binary `metadata` column regardless of whether the Variant value is shredded. +All `variant_value` columns within the Variant must use the same `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. +All fields for a variant, whether shredded or not, must be present in the metadata. Review Comment: I'm a little confused. When the field is not shredded, we will not have metadata for it, right? When it's getting shredded, then it will be like a column and we will generate metadata so it can be used for filtering/pruning? -- 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: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
