raghavyadav01 opened a new pull request, #26836:
URL: https://github.com/apache/datafusion/pull/26836
## Which issue does this PR close?
- Closes #26572.
## Rationale for this change
- With more than one partition, `SELECT t.a, (SELECT max(a) FROM t) AS m ...
ORDER BY t.a, m LIMIT 5` fails with `SanityCheckPlan`.
- `ProjectionExec` does not know `m` is constant, so the child ordering
`[a]` is not seen to satisfy `[a, m]`.
- `ScalarSubqueryExpr` already reports itself as a singleton, but
`is_expr_constant` only treated a `Literal` leaf as constant.
## What changes are included in this PR?
- `is_expr_constant` treats a leaf that reports `SortProperties::Singleton`
as `Uniform(None)`: the same value everywhere, not known yet.
- Two unknown constants are never assumed equal:
- `add_constant` no longer merges two `Uniform(None)` constants into one
class.
- `UNION` turns two `Uniform(None)` sides into `Heterogeneous`.
- Without these, a UNION of two different subqueries lost its sort.
- A volatile expression is never constant, even when its inputs are. This
was already possible on main for a literal argument (`v(1)`).
## What is the testing strategy for this PR?
- New `subquery.slt` cases:
- the query from the issue, plus a variant with a DESC column;
- a UNION of two different subqueries (plan keeps the merge, rows come
back sorted);
- two subqueries in one projection.
- Unit tests in `class.rs`:
- a subquery is `Uniform(None)` and a column is not constant;
- unknown constants stay in separate classes, equal known values still
merge;
- a volatile function of a literal or a subquery is not constant, a stable
one is.
- Without the change, both issue queries fail with the same
`SanityCheckPlan` error.
## Are there any user-facing changes?
- No API changes. Queries that failed now plan, and some plans drop a sort
that a constant made unnecessary.
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