aleksandr-chernousov-db opened a new pull request, #58455:
URL: https://github.com/apache/spark/pull/58455
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### What changes were proposed in this pull request?
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This adds a canonicalization primitive to `VariantBuilder` (in
`common/variant`).
- **`public static Variant canonicalize(Variant v)`** returns a
byte-canonical `Variant`, so two semantically-equal Variants produce
byte-identical `value`/`metadata`. It:
- rebuilds the metadata dictionary sorted by UTF-8 key bytes, deduped,
with unreferenced keys stripped and field ids remapped;
- normalizes scalars: smallest integer width, integer-valued decimals
promoted to the integer encoding, decimal trailing zeros stripped, `-0.0` ->
`+0.0`, non-canonical `NaN` collapsed to the canonical bit pattern, short
strings encoded as `SHORT_STR`. Types with no canonical rule
(null/boolean/date/timestamp/timestamp_ntz/binary/uuid) pass through unchanged.
- A top-level (`pos == 0`) value that is already canonical is returned
unchanged (fast path, below); a sub-variant (`pos != 0`, a view into a parent's
shared arrays) is rebuilt into its standalone canonical form.
- **`public static boolean isCanonical(byte[] value, byte[] metadata)`** is
the read-side predicate whose `true` result guarantees `canonicalize` is a
no-op. It is used as the fast path inside `canonicalize`.
- `VariantUtil` gains `Long.MIN_VALUE`/`Long.MAX_VALUE` `BigDecimal`
constants used by the decimal-to-long promotion check (hoisted out of the
per-call path).
### Why are the changes needed?
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Hash-based grouping and comparison (GROUP BY / DISTINCT / equi-join) bucket
by byte-equality, but two semantically-equal Variants can have different byte
representations:
- {"a": 1, "b": 2} vs {"b": 2, "a": 1}
- different incoming object-key order
- different metadata dictionary order
- non-minimal integer/decimal encodings
- `-0.0` vs `+0.0`
- non-canonical `NaN` bit patterns
- short string stored as `LONG_STR`.
Without a canonical form, such values hash and compare unequal and land in
different buckets, so grouping/deduplication on Variant would produce wrong
results. `canonicalize` maps semantically-equal Variants to identical bytes,
which is the building block for supporting Variant in aggregation/join keys.
This PR introduces only the primitive and its tests. The expression and
analyzer changes that use it will follow in subsequent PRs.
### Does this PR introduce _any_ user-facing change?
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No
### How was this patch tested?
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New tests are introduced in VariantCanonicalizeSuite in this PR to cover all
of the new code
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Generated-By: Claude Opus 4.8
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