perf(optimizer): generic in-place mutable rewrite to obsolete per-rule fast-paths #22616

Is your feature request related to a problem or challenge?

A follow-up to #22583 / #22612.

Several recent optimizer perf PRs have added per-rule fast-paths to dodge wasted work on plans that don't match the rule's shape:

• Optimize logical optimizer: skip map_subqueries + in-place rewriting #22298 — plan_has_subqueries gate around rewrite_with_subqueries
• perf(optimizer): EliminateCrossJoin fast-path for join-free plans #22612 — plan_has_joins gate at the top of EliminateCrossJoin::rewrite
• Likely more to follow on other rules with similar costs

These are useful local wins but @neilconway and @alamb's comments on #22583 point at the deeper issue: the ownership-based TreeNode::rewrite traversal copies the Arc spine on every pass, even when the rule doesn't change anything. A per-rule "did anything here change?" scan is a stop-gap; the real fix is to express tree rewrites in a way where untouched subtrees aren't reallocated to begin with.

@adriangb already laid groundwork in #22298 with a private map_children_mut helper in datafusion/optimizer/src/optimizer.rs that mutates Arc<LogicalPlan> children in place via Arc::make_mut (zero-cost when refcount == 1). That helper currently:

• Is only invoked from rewrite_plan_in_place, which itself is gated on !plan_has_subqueries(plan) — so the in-place path only kicks in for the simpler subquery-free case
• Doesn't recurse into the subquery plans referenced from Expr::ScalarSubquery / InSubquery / Exists / SetComparison
• Is pub(crate) to the optimizer crate, not reachable from individual rules

Describe the solution you'd like

Generalize the in-place rewriting infrastructure so that all logical optimizer rules can opt into it, regardless of ApplyOrder:

1. Extend map_children_mut to also walk subquery plans. Add a sibling map_subquery_plans_mut (or fold it into a single map_children_and_subqueries_mut) so the in-place path works on plans that contain IN (SELECT ...) / EXISTS (...) etc.

2. Expose an in-place traversal entry point usable from rules with ApplyOrder::None. Today EliminateCrossJoin and a couple of other rules drive their own recursion via rewrite_children (which uses ownership-based map_children + map_uncorrelated_subqueries). Give them an in-place equivalent so they can recurse without forcing Arc copies when nothing changes.

3. Decide on the API shape. Two candidates:

   • Add an OptimizerRule::rewrite_in_place(&mut LogicalPlan, ...) -> Result<bool> method alongside the existing by-value rewrite. Rules can opt in; framework falls back to ownership-based otherwise.
   • Or keep the by-value rewrite contract and have the framework do std::mem::take + write-back transparently (similar to how rewrite_plan_in_place already bridges in optimizer.rs:511).

4. Once the framework is in place, remove the per-rule fast-paths. plan_has_subqueries and plan_has_joins become redundant when "no-op rewrites" are already free.

Describe alternatives you've considered

• Keep adding per-rule fast-paths. Easy, contained, and works — but every new rule pays the same cost and we accumulate plan_has_X helpers. Doesn't scale.

• Tackle EliminateCrossJoin specifically with in-place rewrite. Mixed approach: ownership-based for the join restructuring (flatten_join_inputs + find_inner_join), in-place for the non-join recursion. Doable (~300 LOC) but it's a one-off — doesn't help any other rule.

Additional context

• Discussion: perf(optimizer): EliminateCrossJoin walks the full plan tree even when there are no joins #22583 (comments from @neilconway and @alamb)
• Stop-gap PRs: Optimize logical optimizer: skip map_subqueries + in-place rewriting #22298, perf(optimizer): EliminateCrossJoin fast-path for join-free plans #22612
• The map_children_mut implementation lives in datafusion/optimizer/src/optimizer.rs:391
• The gating use-site in rewrite_plan_in_place is at datafusion/optimizer/src/optimizer.rs:498