fix: Handle Unnest LogicalPlan in FederationOptimizer with_new_exprs

[GeorgeLeePatterson]

Summary

Fixes a critical issue where the federation optimizer panics when processing LogicalPlan::Unnest nodes due to DataFusion's inconsistent behavior with expressions() vs with_new_exprs().

Problem

The federation optimizer would crash with:

plan.with_new_exprs(new_expressions, new_inputs)?

because LogicalPlan::Unnest.expressions() returns expressions but with_new_exprs() rejects them in the case of LogicalPlan::Unnest(_).

Solution

• Add special case handling for LogicalPlan::Unnest in optimize_plan_recursively
• Pass empty expressions vector for Unnest plans instead of plan.expressions()
• This follows the same pattern used by other DataFusion optimizers

Testing

• ✅ Added unit test test_federation_optimizer_rule_handles_unnest
• ✅ Added integration example df-unnest.rs demonstrating cross-database federation with unnest
• ✅ Verified fix works with actual federated queries

Checklist

• Tests added/updated
• Example demonstrates the fix
• No breaking changes
• Follows existing code patterns

Next Steps

Will also create a follow up PR with DataFusion with a possible solution to the underlying problem. If that gets merged I will come back and clean this up.

[GeorgeLeePatterson]

@hozan23 @trueleo Checking in to see if you had a chance to review this PR.

[hozan23]

Hello @GeorgeLeePatterson, Thanks for this PR

I think we should consider holding off on merging this for now and instead wait for a proper fix upstream in DataFusion itself. Not a big fan of adding special-case handling directly into the optimizer, it can make the logic harder to read & reason about over time

Let's wait the upstream fix and revisit this once it's available

@trueleo Maybe we can get your thoughts on this as well?

[GeorgeLeePatterson]

@hozan23 Thank you for the reply. That makes sense, I can continue operating on my fork for now in the context of an upcoming crate I am releasing. I have code locally with the fix for DataFusion. If your goal is to benefit from the upstream fix first, then I can shift my focus in that direction and see if I can get the fix merged on the DataFusion side. I appreciate the response and let me know if there's anything else here that is of use. If you do decide to merge it, I will be posting a PR for the DataFusion fix regardless and would be happy to track it to ensure this library gets updated as well.