Fix ordering for UNION ALL over heterogeneous constants#23528
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vadimpiven wants to merge 1 commit into
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Fix ordering for UNION ALL over heterogeneous constants#23528vadimpiven wants to merge 1 commit into
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`add_sort_above` and `add_sort_above_with_distribution` pruned every expression that equivalence properties reported as constant, via `is_expr_constant(..).is_none()`. That treats `AcrossPartitions::Heterogeneous` the same as `AcrossPartitions::Uniform`, which is wrong. A `Heterogeneous` constant is constant *within* each partition but varies *across* partitions (e.g. `sample` in `SELECT 1 AS sample, c FROM t UNION ALL SELECT 2 AS sample, c FROM t`). Both helpers build a partition-preserving `SortExec` whose output is merged across partitions downstream, where such a column is still the leading ordering discriminator. Dropping it silently discards the global ordering, producing non-deterministic output order for `ORDER BY sample, c` once the plan is rooted at a sink / output requirement. Only `AcrossPartitions::Uniform` is globally constant and safe to drop. Introduce `is_uniform_constant` and use it at both prune sites. Adds a regression test that builds a union with a heterogeneous-constant column and asserts the key survives `add_sort_above`.
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@alamb Hi! May I ask you to have a look at this tiny bugfix? |
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Which issue does this PR close?
Rationale for this change
add_sort_aboveandadd_sort_above_with_distributionin physical-optimizer prune every expression thatEquivalenceProperties::is_expr_constantreports as constant.This treats
AcrossPartitions::Heterogeneousthe same asAcrossPartitions::Uniform, which is incorrect.Check an example:
The union reports sample as a
Heterogeneousconstant and so it gets pruned, which breaksORDER BY.What changes are included in this PR?
Helper that activates pruning only for
AcrossPartitions::Uniform, use it at both prune sites (add_sort_aboveandadd_sort_above_with_distribution) soHeterogeneousconstants are retained.Are these changes tested?
Yes, a new unit-test added.
Are there any user-facing changes?
No API changes, just an internal logic bug-fix.