stevomitric opened a new pull request, #58746:
URL: https://github.com/apache/spark/pull/58746
### What changes were proposed in this pull request?
`UnsafeWriter.write(int, TimestampNanosVal)` stores a nanosecond timestamp
as a 16-byte variable-length payload (modeled on `CalendarInterval`) and
reserves that space even for a null value so the slot can be updated in place
later. On the null branch it set the null bit but left the reserved 16 bytes
untouched. The writer's buffer is reused across rows, so a null nanosecond
value inherited whatever bytes the previously written row left in that slot.
Two logically-equal null rows could therefore produce different `UnsafeRow`
byte contents, breaking the invariant that equal rows encode identically -- the
invariant that `UnsafeRow` hashing and equality rely on. This is the root cause
of a nullable nanosecond-timestamp GROUP BY / join key splitting its NULLs
across multiple groups.
The fix zeroes the reserved payload on the null branch, mirroring the
in-place update path (`UnsafeRow.setTimestampNTZNanos` /
`setTimestampLTZNanos`), which already calls
`TimestampNanosRowValues.zeroPayload`.
### Why are the changes needed?
Correctness: a null nanosecond timestamp must encode canonically so that
null grouping/join keys compare and hash identically. Without it, `GROUP BY`
(and any key-based operator) over a nullable `TIMESTAMP_NTZ(p)` /
`TIMESTAMP_LTZ(p)` column can silently scatter NULL rows across several groups.
The array path (`UnsafeArrayWriter`) is unaffected: it writes a null element
through `setNull8Bytes`, which zeroes the element's offset-and-size slot (size
0).
### Does this PR introduce any user-facing change?
No change in a default configuration -- the nanosecond timestamp types are
behind the `spark.sql.timestampNanosTypes.enabled` preview flag (off by
default). With the flag enabled, null nanosecond keys now group and compare
canonically.
### How was this patch tested?
New `UnsafeRowConverterSuite` test asserting that two null nanosecond
projections built after different non-null values are byte-identical, in both
the interpreted and the codegen paths. It fails without this change and passes
with it.
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### What changes were proposed in this pull request?
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