Jubin Soni created SPARK-57738:
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             Summary: ArrowVectorReader guard no longer rejects unsupported 
nanosecond timestamp types over Spark Connect
                 Key: SPARK-57738
                 URL: https://issues.apache.org/jira/browse/SPARK-57738
             Project: Spark
          Issue Type: Bug
          Components: Connect
    Affects Versions: 4.0.3
            Reporter: Jubin Soni


After SPARK-57303 updated {{UpCastRule.canUpCast}} to return {{true}} for 
lossless timestamp-family widening (e.g. {{{}TimestampType -> 
TimestampLTZNanosType(p){}}}), the early-exit safety check in 
{{ArrowVectorReader.applyDefault}} (line 83) no longer rejects requests to read 
a nanosecond-precision timestamp column over Spark Connect.

If a client requests a {{TIMESTAMP_LTZ(p)}} or {{TIMESTAMP_NTZ(p)}} column with 
{{{}p in [7,9]{}}}, the guard passes, but there is no matching {{case}} in the 
{{vector match}} block for nanosecond Arrow vectors. Execution falls through to 
the catch-all {{case _ => throw new RuntimeException("Unsupported Vector 
Type")}} — a confusing crash instead of a clear "not supported" error.

The SPARK-57303 commit message explicitly acknowledges this: 
_"ArrowVectorReader's {{canUpCast}} guard no longer fails fast on a 
micro-vector/nanos-target mismatch; whenever nanos-over-Connect is implemented, 
that PR should add the reader and re-examine this guard."_

*Fix:* Add an explicit rejection in {{applyDefault}} before the {{vector 
match}} for {{{}TimestampLTZNanosType{}}}/{{{}TimestampNTZNanosType{}}} target 
types, until a proper Connect reader is implemented. Relevant file: 
{{sql/connect/common/src/main/scala/org/apache/spark/sql/connect/client/arrow/ArrowVectorReader.scala}}



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