feat!: add element-at expression

[DrakeLin]

🥞 Stacked PR

Use this link to review incremental changes.

• stack/kernel-partition-timezone-expressions [Files changed]
  • stack/kernel-partition-timezone-parsing [Files changed]
    • stack/kernel-partition-timezone-scans [Files changed] ← this PR
      • stack/kernel-partition-timezone-cast [Files changed]
        • stack/kernel-partition-timezone-selective [Files changed]

---

What changes are proposed in this pull request?

Add an ElementAt expression for row-wise Map<String, String> lookup with dynamic keys and rightmost-duplicate semantics. Arrow and DataFusion share the behavior, predicate handling remains conservative, and unsupported FFI visitors receive an explicit unknown node.

This PR affects the following public APIs

Add ElementAtExpression, Expression::element_at, and the ElementAt expression variant.

How was this change tested?

Unit, executor, serialization, predicate, and FFI tests cover dynamic and missing keys, nulls, duplicate keys, and expression traversal.

[codecov]

Codecov Report

❌ Patch coverage is 95.13973% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (728aeb9) to head (3400443).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...src/engine/arrow_expression/evaluate_expression.rs	96.60%	6 Missing and 3 partials ⚠️
kernel/src/timestamp_timezone.rs	90.12%	2 Missing and 6 partials ⚠️
kernel/src/checkpoint/checkpoint_shape.rs	94.02%	1 Missing and 3 partials ⚠️
kernel/src/scan/log_replay.rs	96.66%	4 Missing ⚠️
ffi/src/expressions/kernel_visitor.rs	0.00%	3 Missing ⚠️
kernel/src/expressions/mod.rs	94.33%	3 Missing ⚠️
ffi/src/test_ffi.rs	0.00%	2 Missing ⚠️
kernel/src/plans/proto/convert.rs	94.28%	2 Missing ⚠️
kernel/src/scan/scan_plan.rs	90.90%	0 Missing and 2 partials ⚠️
ffi/src/expressions/engine_visitor.rs	97.95%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3120      +/-   ##
==========================================
+ Coverage   90.08%   90.19%   +0.11%     
==========================================
  Files         248      250       +2     
  Lines       85653    86691    +1038     
  Branches    85653    86691    +1038     
==========================================
+ Hits        77162    78195    +1033     
+ Misses       5684     5676       -8     
- Partials     2807     2820      +13     

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[github-actions]

Benchmark results: ❌ Fail (a benchmark is ≥1.15x slower)

Summary: 🚀 1  ·  ✅ 12  ·  ☑️ 1  ·  🚧 0  ·  ❌ 1

Per-benchmark results (15 rows)

Test	Change	Base	PR
clustered/readMetadataLatestPredicate/serial	✅ 1.00x	96.6±2.16ms	96.9±2.16ms
crcLatest/snapshotLatest	❌ 1.28x slower	9.0±0.10ms	11.5±0.47ms
crcMissing/snapshotLatest	✅ 1.03x faster	28.1±0.84ms	27.4±0.78ms
crcSlightlyStale/snapshotLatest	🚀 1.30x faster	12.7±0.40ms	9.8±0.09ms
crcVeryStale/snapshotLatest	✅ 1.02x faster	18.6±0.99ms	18.2±0.83ms
partitioned/readMetadataLatestPredicate/serial	✅ 1.02x faster	60.2±1.40ms	59.3±1.61ms
v1Checkpoint/readMetadataLatest/serial	☑️ 1.02x slower	12.7±0.31ms	12.9±0.27ms
v1Checkpoint/snapshotLatest	✅ 1.01x faster	870.4±34.71µs	864.3±29.58µs
v2Checkpoint/readMetadataLatest/parallel2	✅ 1.00x	8.9±0.22ms	8.9±0.19ms
v2Checkpoint/readMetadataLatest/serial	✅ 1.00x	15.2±0.32ms	15.2±0.27ms
v2Checkpoint/snapshotLatest	✅ 1.00x	829.5±31.93µs	829.7±35.21µs
wideSchemaJsonStats/readMetadataLatestPredicate/serial	✅ 1.03x faster	74.4±0.66ms	72.0±1.71ms
wideSchemaJsonStats/snapshotLatest	✅ 1.00x	2.4±0.09ms	2.4±0.08ms
wideSchemaStructStats/readMetadataLatestPredicate/serial	✅ 1.03x faster	36.4±0.92ms	35.5±0.70ms
wideSchemaStructStats/snapshotLatest	✅ 1.00x	2.2±0.07ms	2.2±0.06ms

_{Legend: 🚀 ≥1.15x faster  · ✅ faster or unchanged  · ☑️ ≤1.03x slower  · 🚧 1.03x-1.15x slower  · ❌ ≥1.15x slower
Commit: 3400443 · Trigger: auto-push · Tags: base · Updated: 2026-08-14 10:16 PDT}