cashmand opened a new pull request, #52406:
URL: https://github.com/apache/spark/pull/52406
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### What changes were proposed in this pull request?
When writing Variant to Parquet, we want the shredding schema to adapt to
the data being written on a per-file basis. This PR adds a new output writer
that buffers the first few rows before starting the write, then uses the
content of those rows to determine a shredding schema, and only then creates
the Parquet writer with that schema.
The heuristics for determining the shredding schema are currently fairly
simple: if a field appears consistently with a consistent type, we create
`value` and `typed_value`, and if it appears with an inconsistent type, we only
create `value`. We drop fields that occur in less than 10% of sampled rows, and
have an upper bound of 300 total fields (counting `value` and `typed_value`
separately) to avoid creating excessively wide Parquet schemas, which can cause
performance issues.
### Why are the changes needed?
Allows Spark to make use of the [Variant shredding
spec](https://github.com/apache/parquet-format/blob/master/VariantShredding.md)
without requiring the user to manually set a shredding schema.
### Does this PR introduce _any_ user-facing change?
Only if `spark.sql.variant.inferShreddingSchema` and
`spark.sql.variant.writeShredding.enabled` are both set to true. They currently
false by default.
### How was this patch tested?
Unit tests in PR.
### Was this patch authored or co-authored using generative AI tooling?
No.
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