fix: unparse columns of stacked pushdown projections unqualified (#23176)

## Which issue does this PR close?

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- Closes #23138 .

## Rationale for this change

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For an optimized plan, common subexpression elimination can leave a
`SubqueryAlias` over two stacked `Projection`s (it factors a shared
expression
into an extra inner projection). The plan is correct; the unparser just
cannot
render it accurately.

When unparsing such a subquery, `unparse_table_scan_pushdown` pushes the
subquery alias down to the aliased table scan and rebases each
projection's
columns to that alias. That is correct for the projection directly above
the
scan, but an *outer* stacked projection sits over a derived table where
the
alias is no longer in scope, so rebasing its qualified pass-through
columns
emits references the surrounding query cannot resolve.

For the reproducer in the issue the generated SQL is (note the middle
`"o"."order_id"`):

```sql
... INNER JOIN (
  SELECT "o"."order_id", CASE WHEN "__common_expr_1" ... END AS "discount_pct_2"
  FROM (SELECT CAST("o"."discount_pct" ...) AS "__common_expr_1", "o"."order_id"
        FROM "warehouse"."main"."orders" AS "o")
) AS "o" ON ...
```

DuckDB rejects it with Binder Error: Referenced table "o" not found!,
because
"o" is only the base-table alias one level deeper, not visible in the
intermediate derived table.

## What changes are included in this PR?

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In unparse_table_scan_pushdown's Projection arm, detect the stacked
case:
if the recursive pushdown result is itself a Projection, this projection
sits over a derived table rather than directly over the aliased scan.
In that case, strip the table qualifier from the projection's
pass-through
columns so they reference the derived table's output unqualified,
instead of
being rebased to the (out-of-scope) scan alias. The projection is built
directly (Projection::try_new) so the unqualified columns are not
re-normalized back to the alias.
The projection directly above the scan is unchanged and still rebases to
the
alias.
After the fix the middle column is unqualified and the SQL is valid:

```sql
... INNER JOIN (
  SELECT "order_id", CASE WHEN "__common_expr_1" ... END AS "discount_pct_2"
  FROM (SELECT CAST("o"."discount_pct" ...) AS "__common_expr_1", "o"."order_id"
        FROM "warehouse"."main"."orders" AS "o")
) AS "o" ON ...
```

## Are these changes tested?

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New test `optimized_duckdb_unparse_qualifies_nested_passthrough_column`
in
`datafusion/core/tests/sql/unparser.rs`, asserting the intermediate
derived
table no longer carries the out-of-scope alias.

## Are there any user-facing changes?

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updated before approving the PR.
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No public API changes.

---------

Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>

Author: Phoenix500526

datafusion/core/tests/sql/unparser.rs

@@ -343,6 +343,62 @@ async fn optimized_duckdb_unparse_preserves_derived_table_scope() -> Result<()>
     Ok(())
 }
 
+// https://github.com/apache/datafusion/issues/23138
+//
+// CSE on `coalesce(discount_pct, 0)` factors a shared CAST into an extra inner
+// projection, so `SubqueryAlias: o` ends up over two stacked projections. When
+// the unparser renders that as nested derived tables it must qualify the
+// pass-through `order_id` with a name in scope at each level -- it must not
+// rebase it to the outer subquery alias `o`, which is not visible inside the
+// inner derived table.
+const ISSUE_23138_QUERY: &str = r#"
+SELECT * FROM
+(
+    SELECT order_id FROM "warehouse"."main"."order_items"
+) oi
+JOIN (
+    SELECT order_id, coalesce(discount_pct, 0) AS discount_pct_2
+    FROM "warehouse"."main"."orders"
+) o USING (order_id)
+"#;
+
+#[tokio::test]
+async fn optimized_duckdb_unparse_qualifies_nested_passthrough_column() -> Result<()> {
+    let ctx = issue_22961_context()?;
+    let plan = ctx.sql(ISSUE_23138_QUERY).await?.into_optimized_plan()?;
+    let dialect = DuckDBDialect::new();
+    let unparser = Unparser::new(&dialect);
+    let sql = unparser.plan_to_sql(&plan)?.to_string();
+
+    // The intermediate derived table has no `o` in scope, so the pass-through
+    // `order_id` must be unqualified there, not rebased to the subquery alias
+    // `o` (which is only the base-table alias one level deeper). The bug emitted
+    // `"o"."order_id"` inside that derived table; the fix emits a bare column.
+    let expected = concat!(
+        r#"SELECT "o"."order_id", "o"."discount_pct_2" "#,
+        r#"FROM "warehouse"."main"."order_items" AS "oi" "#,
+        r#"INNER JOIN (SELECT "order_id", "#,
+        r#"CASE WHEN "__common_expr_1" IS NOT NULL "#,
+        r#"THEN "__common_expr_1" ELSE 0.00 END AS "discount_pct_2" "#,
+        r#"FROM (SELECT CAST("o"."discount_pct" AS DECIMAL(22,2)) "#,
+        r#"AS "__common_expr_1", "o"."order_id" "#,
+        r#"FROM "warehouse"."main"."orders" AS "o")) AS "o" "#,
+        r#"ON "oi"."order_id" = "o"."order_id""#,
+    );
+    assert_eq!(sql, expected);
+
+    assert!(
+        sql.contains(r#"(SELECT "order_id", CASE WHEN"#),
+        "pass-through order_id should be unqualified in derived table: {sql}"
+    );
+    assert!(
+        !sql.contains(r#"(SELECT "o"."order_id", CASE WHEN"#),
+        "derived table must not reference out-of-scope alias o: {sql}"
+    );
+
+    Ok(())
+}
+
 /// The outcome of running a single roundtrip test.
 ///
 /// A successful test produces [`TestCaseResult::Success`].

datafusion/sql/src/unparser/plan.rs

@@ -1992,6 +1992,21 @@ impl Unparser<'_> {
         Ok(Some(relation))
     }
 
+    /// Strip the table qualifier from every column in a pushdown pass-through
+    /// projection expression, so it resolves against the unnamed derived table
+    /// rendered for the inner pushdown projection rather than a deeper table
+    /// alias that is out of scope at this nesting level.
+    fn strip_pushdown_column_qualifiers(expr: Expr) -> Result<Expr> {
+        expr.transform(|e| match e {
+            Expr::Column(mut column) => {
+                column.relation = None;
+                Ok(Transformed::yes(Expr::Column(column)))
+            }
+            other => Ok(Transformed::no(other)),
+        })
+        .data()
+    }
+
     /// Try to unparse a table scan with pushdown operations into a new subquery plan.
     /// If the table scan is without any pushdown operations, return None.
     fn unparse_table_scan_pushdown(
@@ -2121,6 +2136,33 @@ impl Unparser<'_> {
                     alias.clone(),
                     already_projected,
                 )? {
+                    // The pushed-down scan alias is only in scope for the
+                    // projection directly above the aliased table scan. `plan`
+                    // is the result of pushing the alias further down: if it is
+                    // itself a `Projection`, the input was another projection
+                    // (e.g. common subexpression elimination stacked one), so
+                    // this projection sits over a derived table rather than
+                    // directly over the aliased scan, and the alias is out of
+                    // scope here. Its qualified pass-through columns must then
+                    // reference the derived table's output unqualified instead
+                    // of being rebased to the alias. Build it directly so the
+                    // unqualified columns are not re-normalized back to the
+                    // alias. (Otherwise `plan` is the scan-derived plan and we
+                    // fall through to rebase to the alias, correct one level
+                    // above the scan.)
+                    if alias.is_some() && matches!(plan, LogicalPlan::Projection(_)) {
+                        let exprs = projection
+                            .expr
+                            .iter()
+                            .cloned()
+                            .map(Self::strip_pushdown_column_qualifiers)
+                            .collect::<Result<Vec<_>>>()?;
+                        return Ok(Some(LogicalPlan::Projection(Projection::try_new(
+                            exprs,
+                            Arc::new(plan),
+                        )?)));
+                    }
+
                     let exprs = if alias.is_some() {
                         let mut alias_rewriter =
                             alias.as_ref().map(|alias_name| TableAliasRewriter {