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[branch-56] fix: infer UNION output types before analysis (backport #27139) - #27178
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## Which issue does this PR close? - Closes apache#27023. - Part of apache#14380. UNION inputs still need analyzer casts. ## Rationale for this change `DataFrame::schema()` previously reported the first UNION input's type while execution used the common type. UNION construction now infers the common output type. ## What changes are included in this PR? - Infer UNION and CASE result types before analysis, with cast insertion deferred to the analyzer. - Derive final UNION types from actual inputs with the shared `coerce_union_type` helper. - A branch-56 backport remains a maintainer decision because public inference behavior and one executed Arrow type change. Synthetic nested CASE planning (`ci-bench`, SQL through physical planning): | Depth | Main (µs) | Patch (µs) | Extra (µs) | | --- | --- | --- | --- | | 8 | 101.2 | 105.5 | +4.3 | | 32 | 270.1 | 317.6 | +47.5 | | 64 | 502.8 | 684.0 | +181.2 | These three points suggest roughly quadratic extra work. Extrapolating that trend to depth 1,000 gives about 45 ms of extra planning. The benchmark did not measure depth 1,000. ## What is the testing strategy for this PR? Focused core and expression tests (316), 64 doctests, five SLT files, all-features Clippy, Rustdoc, and formatting pass. ## Are there any user-facing changes? In this query, `x` changes from `Dictionary(Int32, Utf8)` to `Utf8`; decoded values remain unchanged. Rustdoc describes UNION and CASE schema inference. ```sql SELECT 1 AS y UNION ALL BY NAME SELECT arrow_cast('a', 'Dictionary(Int32, Utf8)') AS x; ``` (cherry picked from commit 13bd9d1)
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Looks like a clean cherry-pick, approved pending CI. Thanks @alamb!
Codecov Report❌ Patch coverage is
Flags with carried forward coverage won't be shown. Click here to find out more. @@ Coverage Diff @@
## branch-56 #27178 +/- ##
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Coverage 83.26% 83.26%
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Files 1169 1169
Lines 474209 474299 +90
Branches 474209 474299 +90
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+ Hits 394853 394935 +82
- Misses 55458 55459 +1
- Partials 23898 23905 +7
... and 5 files with indirect coverage changes 🚀 New features to boost your workflow:
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Which issue does this PR close?
branch-5656.0.0(Oct/Nov 2026) #24461