Phoenix500526 opened a new pull request, #22998:
URL: https://github.com/apache/datafusion/pull/22998
## Which issue does this PR close?
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- Closes #22881
## Rationale for this change
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A `USING` / `NATURAL` join exposes its join key as a single merged column
whose value, per the SQL standard, is `COALESCE(left.key, right.key)`.
DataFusion resolved an unqualified reference to that merged key to the left
column unconditionally.
For `RIGHT` and `FULL` joins the left key is NULL-padded on rows that exist
only on the right, so the merged key came out NULL instead of the value that is
actually present. The wrong NULL is silent and propagates into `GROUP BY`,
`ORDER BY` (changing row order) and `SELECT *`, corrupting results; `WHERE` on
the merged key additionally failed with an "ambiguous reference" error. `INNER`
/ `LEFT` are unaffected, since their left key is never NULL-padded.
```sql
create table a(k int, x int) as values (1, 10), (2, 20), (3, 30);
create table b(k int, y int) as values (2, 200), (3, 300), (4, 400);
select k from a right join b using (k) order by k nulls last;
-- before: 2, 3, NULL after: 2, 3, 4
```
## What changes are included in this PR?
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The unqualified merged key of a `RIGHT` / `FULL`` USING` / `NATURAL` join
now resolves to `COALESCE(left, right)` everywhere it can be referenced:
* column normalization (`SELECT`, `ORDER BY`, `GROUP BY`, `HAVING`,
`QUALIFY`);
* `WHERE` predicates — previously rejected as ambiguous; they now resolve
against the join's real USING columns;
* wildcard expansion (`SELECT *`).
Mechanics:
* a new `LogicalPlan::outer_using_key_pairs()` returns the `(left, right)`
key pairs of `RIGHT` / `FULL` `USING` / `NATURAL` joins;
* the merged key is materialized as `CASE WHEN left IS NOT NULL THEN left
ELSE right END` — the exact form coalesce is simplified to, so it can be built
in `datafusion-expr` without depending on the functions crate — aliased to the
key name so the output column keeps its name.
`INNER` / `LEFT` joins and qualified access (a.k / b.k) are left unchanged.
## Are these changes tested?
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Yes. A new sqllogictest file `join_using_merged_key.slt` covers the merged
key for `RIGHT` / `FULL` / `NATURAL` joins across `SELECT`, `SELECT *`, `WHERE`
and `ORDER BY`, with `INNER` / `LEFT`, qualified `a.k` / `b.k` access, and the
explicit `coalesce(a.k, b.k) ... ON` form as regression guards.
The full sqllogictest suite and the `datafusion-common` /`datafusion-expr` /
`datafusion-sql` unit tests pass with no regressions.
## Are there any user-facing changes?
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Yes. For `RIGHT` / `FULL` `USING` / `NATURAL` joins, the merged join key now
returns `COALESCE(left, right)` (the value from whichever side is present)
instead of `NULL` for rows that exist only on the right, and referencing the
merged key in `WHERE` no longer raises an ambiguous-reference error. Queries
that do not use `RIGHT` / `FULL` `USING` / `NATURAL` joins are unaffected.
No breaking public API changes (the change only adds new public items).
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