fix: keep EliminateGroupByConstant from requalifying group expressions

[adriangb]

Which issue does this PR close?

No issue filed. The reproduction is below.

Rationale for this change

EliminateGroupByConstant re-plans the original group expressions over the new Aggregate with LogicalPlanBuilder::project. Column normalization falls back to the input schema of the aggregate. Thus an unqualified group expression, such as an extracted leaf __datafusion_extracted_1, becomes m.__datafusion_extracted_1 when the input is a SubqueryAlias: m. The new aggregate does not output that name, and the rule fails:

Optimizer rule 'eliminate_group_by_constant' failed
caused by
Schema error: No field named m.__datafusion_extracted_1.

The input plan is valid, and the rule turns it into an invalid one. We reach it when we optimize a plan, bind its parameters with replace_params_with_values, and optimize again. The bound value makes a group key redundant only after ExtractLeafExpressions has rewritten the plan. In a single optimize call, EliminateGroupByConstant runs before ExtractLeafExpressions in each pass, so I could not reproduce it there. Since #26184, the arguments of filtered aggregates no longer go into a common-expression projection between the aggregate and the alias, so the aggregate can sit directly on the alias and the problem shows.

let ctx = SessionContext::new();
ctx.sql("CREATE TABLE t AS SELECT * FROM (VALUES (TIMESTAMP '2026-01-01T00:00:00', named_struct('a', 'x'), 1)) AS v(ts, s, x)")
    .await?.collect().await?;
let scan = ctx.table("t").await?.into_unoptimized_plan();
let plan = LogicalPlanBuilder::from(scan)
    .alias("m")?
    .aggregate(
        vec![
            date_bin(
                cast(placeholder("$1"), DataType::Interval(IntervalUnit::MonthDayNano)),
                col("ts"),
                lit(ScalarValue::TimestampNanosecond(Some(0), None)),
            ),
            get_field(col("s"), "a").alias("c"),
            col("ts"),
        ],
        vec![
            sum(col("x") + lit(1)).filter(col("x").gt(lit(0))).build()?,
            max(col("x") + lit(1)).filter(col("x").lt(lit(5))).build()?,
        ],
    )?
    .build()?;
let state = ctx.state();
let plan = state.optimize(&plan)?;
let plan = plan.replace_params_with_values(&ParamValues::List(vec![
    ScalarValue::Utf8(Some("1 minute".into())).into(),
]))?;
state.optimize(&plan)?; // fails with the error above

What changes are included in this PR?

• The rule builds the projection with Projection::try_new and does not re-plan the group expressions. A kept key or an aggregate becomes a column of the schema of the new aggregate. A redundant key keeps its expression, which reads only bare GROUP BY columns that the new aggregate outputs unchanged. An item gets an alias only when its name differs from the original output field, so existing plans do not change.
• The rule leaves an aggregate with a GroupingSet unchanged, because its output fields do not map one to one to group_expr.

What is the testing strategy for this PR?

• New unit test test_eliminate_with_unqualified_reference_to_qualified_input. It fails without the change, with the same schema error.
• cargo test -p datafusion-optimizer and cargo test -p datafusion-sqllogictest --test sqllogictests pass. No existing plan changes.

Are there any user-facing changes?

No API changes. A plan that failed in this rule now optimizes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.70588% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.89%. Comparing base (e732212) to head (2a7c2fa).
⚠️ Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
...usion/optimizer/src/eliminate_group_by_constant.rs	89.70%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26356      +/-   ##
==========================================
+ Coverage   82.83%   82.89%   +0.05%     
==========================================
  Files        1151     1153       +2     
  Lines      455332   457542    +2210     
  Branches   455332   457542    +2210     
==========================================
+ Hits       377165   379257    +2092     
- Misses      55262    55272      +10     
- Partials    22905    23013     +108     

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[Copilot]

🟡 Changes recommended

Removing aliases from redundant group keys discards their output field metadata.

1 open finding

• Preserve metadata for redundant group-key aliases · New

What changed in this PR

Fixes EliminateGroupByConstant incorrectly requalifying group expressions when rebuilding an aggregate’s output projection.

Changes:

• Builds the projection directly from aggregate output columns.
• Leaves grouping-set aggregates unchanged.
• Adds a regression test for unqualified references to qualified inputs.

File	Description
datafusion/​optimizer/​src/​eliminate_group_by_constant.rs	Updates projection construction, guards grouping sets, and adds regression coverage.

🧠 Review effort: Balanced

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[Copilot]

🟢 Approval recommended

The focused rewrite addresses the qualification failure with regression coverage and no identified blocking issues; tests were not rerun during review.

0 open findings

1 resolved since last review

• Preserve metadata for redundant group-key aliases

🧠 Review effort: Balanced