ORC, Flink: Avoid redundant java.time allocations when reading ORC timestamps

[wombatu-kun]

The generic and Flink ORC timestamp value readers build throwaway java.time objects on every value. Reading a timestamp/timestamptz column went through Instant.ofEpochSecond(...).atOffset(ZoneOffset.UTC)[...], which allocates, per value: a transient Instant; an OffsetDateTime that is immediately discarded (for the readers that ultimately return a LocalDateTime/TimestampData); and, less obviously, a ZoneRules object plus a small backing array, because instant.atOffset(ZoneOffset.UTC) routes through OffsetDateTime.ofInstant(instant, zone), which calls zone.getRules(), and ZoneOffset.getRules() returns a freshly allocated ZoneRules on every call.

This replaces those expressions with direct constructions that pass the ZoneOffset straight to the factory methods and never call getRules(). For example, in GenericOrcReaders:

// TimestampReader (-> LocalDateTime)
-  Instant.ofEpochSecond(Math.floorDiv(tcv.time[row], 1_000), tcv.nanos[row])
-      .atOffset(ZoneOffset.UTC)
-      .toLocalDateTime();
+  LocalDateTime.ofEpochSecond(Math.floorDiv(tcv.time[row], 1_000), tcv.nanos[row], ZoneOffset.UTC);

The timestamptz readers follow the same shape (OffsetDateTime.of(LocalDateTime.ofEpochSecond(...), UTC) in the generic reader; TimestampData.fromInstant(Instant.ofEpochSecond(...)) in Flink). The Flink TimestampTzReader additionally did Instant.ofEpochSecond(...).atOffset(UTC).toInstant(), an identity round-trip back to an Instant, which is now removed.

All rewrites are behavior-preserving. instant.atOffset(UTC).toLocalDateTime() is exactly what LocalDateTime.ofEpochSecond(seconds, nanoOfSecond, UTC) produces, and the readers always pass tcv.nanos[row], which is in [0, 1_000_000_000) (it carries java.sql.Timestamp.getNanos() semantics; the whole second and the sign live in tcv.time and are split out with Math.floorDiv), so the nanoOfSecond range contract of LocalDateTime.ofEpochSecond is always satisfied.

Affected readers:

• iceberg-orc GenericOrcReaders (TimestampReader, TimestampTzReader) - the generic Record ORC path, used by IcebergGenerics and by delete-file reads via BaseDeleteLoader.
• iceberg-flink FlinkOrcReaders for v2.1 (TimestampReader, TimestampTzReader) - the Flink ORC scan path. The change is identical for v1.20 and v2.0; a backport PR will follow.

Benchmarks (JMH, JDK 17, SingleShotTime, gc profiler; allocation is gc.alloc.rate.norm, which is deterministic):

Generic Record read, 2.5M rows x 8 timestamp columns (20M values/op):

Metric	Before	After
time	2.431 s/op	1.983 s/op (-18%)
allocation	7.434 GB/op	5.034 GB/op (-32%, -120 B/value)

Flink TimestampData readers, 10.24M values/op:

Reader	time (before -> after)	allocation (before -> after)
timestamp	0.358 -> 0.291 s/op	144 -> 24 B/value (-83%)
timestamptz	0.380 -> 0.070 s/op	168 -> 24 B/value (-86%)

After the change both Flink readers bottom out at the 24-byte TimestampData result, because simplifying the expression also lets the JIT scalar-replace the residual LocalDateTime/LocalDate/LocalTime (in the original, the atOffset detour and the escaping ZoneRules defeated escape analysis).

Testing: covered by existing round-trip tests - TestGenericData (parameterized over timestamp, timestamptz, timestamp_ns, timestamptz_ns, including negative epochs and a cross-timezone write/read) and TestFlinkOrcReaderWriter; both pass with the change.

---

AI Disclosure

• Model: Claude Opus 4.8
• Platform/Tool: Claude Code
• Human Oversight: fully reviewed
• Prompt Summary: Remove the redundant per-value Instant and OffsetDateTime allocations from the generic and Flink ORC timestamp readers.

[github-actions]

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[wombatu-kun]

no stale