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fix: infer UNION output types before analysis - #27139
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alamb merged 2 commits intoNov 6, 2026
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kumarUjjawal
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I was just about to post a fix too 😭 |
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Should've taken the issue yesterday |
alamb
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Nov 5, 2026
| /// alone is not enough here -- it reflects the raw, pre-`Analyzer` plan (see | ||
| /// `SessionState::create_logical_plan`), which for a `UNION` still has the | ||
| /// first leg's un-coerced type. | ||
| /// This test checks the schema after the Analyzer and Optimizer run, |
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thank you for trimming the AI verbosity
| in this section pertains to features and changes that have already been merged | ||
| to the main branch and are awaiting release in this version. | ||
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| ### UNION and CASE infer common output types before analysis |
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This is a bug fix. Why does it need a upgrade guide entry?
| `datafusion_expr::type_coercion::other::coerce_union_type` is a new helper | ||
| that infers one UNION column type from input fields. | ||
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| For `UNION BY NAME`, missing columns participate as `NULL` during type |
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If we want to document the behavior of how schema resolution works, it should be in the code documentation I think
| .transpose()?; | ||
| // If the branches have no common type, keep the old provisional | ||
| // type. The analyzer reports the error. | ||
| Ok( |
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we could probably make this more concse / less indented with some intermediate variables
| pub fn coerce_union_schema(inputs: &[Arc<LogicalPlan>]) -> Result<DFSchema> { | ||
| coerce_union_schema_with_schema(&inputs[1..], inputs[0].schema()) | ||
| let Some(first) = inputs.first() else { | ||
| return plan_err!("UNION requires at least one input"); |
| } | ||
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| #[tokio::test] | ||
| async fn union_coercion_preserves_values() -> Result<()> { |
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I feel like most of these tests can be written as .slt tests (but better). I recommend leaving some tests here that cover the expected schema case but don't run the query -- instead run the queries as a .slt tests
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Thank you for running with this @kumarUjjawal |
sdf-jkl
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Nov 5, 2026
Incorporate deferred CASE/UNION coercion errors, dictionary coercion from actual inputs, and an empty-input guard, informed by: apache#27139 Keep regression coverage in SQL and document the dictionary output-type change for UNION BY NAME.
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Thank you @kumarUjjawal |
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Which issue does this PR close?
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?
coerce_union_typehelper.Synthetic nested CASE planning (
ci-bench, SQL through physical planning):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,
xchanges fromDictionary(Int32, Utf8)toUtf8; decoded values remain unchanged. Rustdoc describes UNION and CASE schema inference.