fix(cubesql): Handle array-typed columns in streaming SQL mode

[MazterQyou]

Check List

• Tests have been run in packages where changes have been made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Description of Changes Made

This PR fixes handling array-typed columns in streaming mode. Related test is included.

This behavior is not documented and unsupported, so it should not be relied upon. The fix is being landed to bring streaming and non-streaming modes in line.

[claude]

Claude finished @MazterQyou's task in 1m 34s —— View job

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Claude PR Review

Reviewed the four-file change. Overall this is a focused, well-scoped fix that brings streaming-mode behavior in line with the non-streaming path (both now JSON-stringify non-scalar values) and is backed by a unit test in scan.rs plus an integration test in the Postgres driver.

Verdict: LGTM — 0 high, 1 medium, 2 low.

Details

Findings

Medium — PostgresDriver.ts may have silently fixed a latent enum-mapping bug

In packages/cubejs-postgres-driver/src/PostgresDriver.ts:304-316, adding AS typname aliases the CASE expression so current.typname is now populated. Before this change the result column for the CASE branch had no explicit name, so depending on driver behavior current.typname was likely undefined for every row — meaning the existing enum (typcategory = 'E') mapping to varchar may not have been working at all, and userDefinedTypes was being populated with { [oid]: undefined }. Worth confirming and calling out in the PR description, since this silently fixes more than just arrays.

Low — stream.rs: JsObject branch is a broad catch-all

packages/cubejs-backend-native/src/stream.rs:322-326 will JSON.stringify any JsObject not already matched. The branch ordering (after JsDate) is correct so dates still error rather than serializing, but Buffers / typed arrays / Maps will silently produce {"type":"Buffer",...} / {} / {} rather than a clear error. For the documented streaming-mode use case this is fine, but a brief comment noting the intent ("treat any unknown object as JSON for parity with the non-streaming path") would help the next reader.

Low — stream.rs: js_value_to_json_string re-resolves JSON.stringify on every call

packages/cubejs-backend-native/src/stream.rs:264-283 looks up JSON and JSON.stringify on the global for every array/object cell. For chunks with many array columns this is a non-trivial overhead. Caching the JsFunction handle (e.g. on JsValueObject or a per-chunk scope) would avoid the repeated global lookups. Not a blocker; mention in case throughput matters here.

Notes / Praise

• Replacing the .unwrap() at the old transform_response call site with a proper throw_error (stream.rs:349-353) is a nice correctness improvement on its own — previously a malformed chunk would crash the Node process.
• The scan.rs unit test (convert_transport_response_serializes_non_scalar_values_to_json_strings) is well-targeted: covers both array and object shapes, plus the null case.
• Postgres integration test exercises both text[] and int[] — good coverage of the array category.
• Conventional commit message and CHANGELOG-friendly description.

• [branch](https://github.com/cube-js/cube/tree/cubesql/streaming-mode-array-fix)

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.20%. Comparing base (e317395) to head (a04208e).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
rust/cubesql/cubesql/src/compile/engine/df/scan.rs	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11149      +/-   ##
==========================================
- Coverage   83.68%   79.20%   -4.49%     
==========================================
  Files         257      474     +217     
  Lines       79384    96934   +17550     
  Branches        0     3563    +3563     
==========================================
+ Hits        66434    76774   +10340     
- Misses      12950    19642    +6692     
- Partials        0      518     +518     

Flag	Coverage Δ
cube-backend	58.80% <ø> (?)
cubesql	83.69% <95.45%> (+0.01%)	⬆️

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