[jira] [Created] (SPARK-57461) Cast between nanosecond-precision timestamp types with different fractional precision

["Max Gekk (Jira)" <ji...@apache.org>]

Max Gekk created SPARK-57461:
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             Summary: Cast between nanosecond-precision timestamp types with different fractional precision
                 Key: SPARK-57461
                 URL: https://issues.apache.org/jira/browse/SPARK-57461
             Project: Spark
          Issue Type: Sub-task
          Components: SQL
    Affects Versions: 5.0.0
            Reporter: Max Gekk


Umbrella: SPARK-56822 (Timestamps with nanosecond precision).

Support casting a nanosecond-precision timestamp to the same family with a different fractional precision, i.e. TIMESTAMP_NTZ(p1) -> TIMESTAMP_NTZ(p2) and TIMESTAMP_LTZ(p1) -> TIMESTAMP_LTZ(p2) for p1 != p2 (p in 7-9).

Today Cast.scala has no canCast / canAnsiCast rule for an already-nanosecond source to a nanosecond target with a different precision, and the runtime helpers castToTimestampNTZNanos / castToTimestampLTZNanos only handle String / microsecond / DATE sources - their precision parameter is used only for string parsing. As a result CAST(ts7 AS TIMESTAMP_NTZ(9)) (and narrowing CAST(ts9 AS TIMESTAMP_NTZ(7))) fails the type check.

Scope:
- Add canCast / canAnsiCast rules for nanos(p1) -> nanos(p2) within the same family (NTZ->NTZ, LTZ->LTZ).
- Implement interpreted and codegen runtime: widening keeps the value; narrowing truncates (or rounds per FSP rules, aligned with microsecond cast behavior) the sub-microsecond nanos component to the target precision.
- Decide store-assignment / up-cast behavior (coordinate with SPARK-57303): widening p1<p2 as up-cast vs narrowing requiring explicit cast.
- Tests in CastSuite / AnsiCastSuite covering all 7-9 combinations, NTZ and LTZ, ANSI and non-ANSI, including boundary nanos values.



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