Skip map_expressions rebuild for Extension nodes with empty expressions

[zhuqi-lucas]

Which issue does this PR close?

Closes #21700.

Rationale for this change

When an Extension node has no expressions, map_expressions was still cloning all inputs and calling with_exprs_and_inputs to reconstruct the node — wasted work since there are no expressions to transform. This is common for Extension nodes like view matching candidates that carry multiple children but no expressions.

What changes are included in this PR?

1. Code change (datafusion/expr/src/logical_plan/tree_node.rs): Add early return when node.expressions() is empty, skipping the clone + rebuild path.

2. Micro-benchmark (datafusion/expr/benches/map_expressions.rs): Criterion benchmark comparing map_expressions on Extension nodes with and without expressions, varying the number of children (1, 3, 5, 10).

Are these changes tested?

Yes — existing tests pass, and the new benchmark validates the optimization.

Benchmark results:

Children	no_expr (optimized)	with_expr (rebuild)	Speedup
1	24 ns	167 ns	7x
3	23 ns	192 ns	8x
5	23 ns	181 ns	8x
10	24 ns	216 ns	9x

The no_expr path is constant time regardless of children count.

In a real optimizer pipeline (~15 rules × 5-child Extension), this saves ~2.4 us per optimization pass.

Next step

A more general optimization would be to change UserDefinedLogicalNode::expressions() to return references (&[Expr]) instead of cloned Vec<Expr>, and only clone + rebuild when the transform actually modifies an expression. This would avoid the clone + with_exprs_and_inputs rebuild even for non-empty expression lists when the transform is a no-op. Added a TODO comment in the code for this direction. This would be a larger API change, so the empty-expressions shortcut is a pragmatic first step.

Are there any user-facing changes?

No — purely internal optimization. No API changes.

[Copilot]

Pull request overview

This PR optimizes LogicalPlan::map_expressions for Extension nodes that expose no expressions, avoiding unnecessary cloning/rebuild work, and adds a Criterion micro-benchmark to quantify the improvement.

Changes:

• Add an early return in map_expressions for Extension nodes when node.expressions() is empty.
• Add a Criterion benchmark measuring map_expressions behavior for Extension nodes with/without expressions across varying child counts.
• Register the new benchmark in datafusion-expr’s dev-dependencies and bench configuration.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 4 comments.

File	Description
datafusion/expr/src/logical_plan/tree_node.rs	Skip rebuilding Extension nodes in map_expressions when there are no expressions to transform.
datafusion/expr/benches/map_expressions.rs	Add micro-benchmark for map_expressions on Extension nodes with/without expressions.
datafusion/expr/Cargo.toml	Add Criterion dev-dependency and register the new bench target.
Cargo.lock	Lockfile updates due to added Criterion dependency/versioning.

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