perf: gather Linear-mode window input more efficiently

[neilconway]

Which issue does this PR close?

• Part of BoundedWindowAggExec in Linear mode is slow for many-partitions #23982

Rationale for this change

LinearSearch::evaluate_partition_batches issued one take_record_batch call per partition present in the input batch. For batches with many partitions, this is inefficient.

Instead, we can build a Vec of the batch's row indices that groups the rows by partition, gather all rows with a single take_record_batch call, and hand each partition a slice of the result. Batches that contain a single partition skip the gather entirely.

Memory caveat: the emitted slices share the gathered batch's buffers. A partition that never receives rows again retains its slice and therefore pins the gathered batch's buffers (up to one input batch worth of memory per input batch in the worst case). This could be addressed, e.g., with a compaction pass to copy long-lived slices into owned buffers, but I have omitted that for now.

Benchmarks: (using #24032)

• linear 100 partitions: 44.3 ms -> 44.1 ms (within noise)
• linear 10000 partitions: 199.8 ms -> 170.1 ms (-14.9%)
• linear sparse 32768 partitions: 224.6 ms -> 205.3 ms (-8.6%)
• linear rows 10000 partitions: 169.0 ms -> 142.0 ms (-15.9%)
• linear multi 10000 partitions: 295.9 ms -> 268.2 ms (-9.4%)
• sorted 10000 partitions: 34.0 ms -> 34.5 ms (+1.3%; unchanged code path)

What changes are included in this PR?

• Rewrite get_per_partition_indices and rename to compute_partition_permutation
• Add focused unit test for computing partition permutations correctly

Are these changes tested?

Yes, new test added.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.52381% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.85%. Comparing base (dbcb5c0) to head (f12b81a).

Files with missing lines	Patch %	Lines
...ysical-plan/src/windows/bounded_window_agg_exec.rs	89.52%	4 Missing and 7 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24034   +/-   ##
=======================================
  Coverage   80.85%   80.85%           
=======================================
  Files        1101     1101           
  Lines      374933   375018   +85     
  Branches   374933   375018   +85     
=======================================
+ Hits       303166   303237   +71     
- Misses      53671    53679    +8     
- Partials    18096    18102    +6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[alamb]

FYI @avantgardnerio / @2010YOUY01 who were perhaps interested in window function performance

[Dandandan]

Nice result for a (relatively) small change!

[neilconway]

@Dandandan Thanks for the reviews!