Merge sorted files in IcebergDataSource only when something above the scan uses their order #72

Follow-up to #55 (comment).

With iceberg.execution.preserve_data_ordering on, TableProvider::scan lists the data files, and if at most iceberg.execution.max_merge_files_per_partition share a sort order, IcebergDataSource reports it and open() k-way merges one reader per file, with every file open, on one thread. It merges even when nothing above the scan uses the order:

• a SELECT *, a GROUP BY on another column, or a hash join;
• ORDER BY id DESC, which keeps its SortExec above the merge;
• ORDER BY id on a nullable key, because Iceberg sorts ascending fields NULLS FIRST by default while DataFusion's ORDER BY means NULLS LAST, so the sort stays and we pay for both.

TableProvider::scan can't tell whether anything above will use the order.

What DataFusion 55 offers. The PushdownSort rule (on by default through datafusion.optimizer.enable_sort_pushdown) offers each SortExec's order to its input through try_pushdown_sort. Exact removes the sort, Inexact keeps it over a rebuilt input, and Unsupported changes nothing. DataSourceExec passes the request to the source, and CooperativeExec, ProjectionExec and order-keeping RepartitionExec pass it down, but FilterExec doesn't, so a WHERE clause blocks it. The call is synchronous, so answering it means keeping the file listing in TableProvider::scan.

Sort-merge joins. As @mbutrovich pointed out, a design that merges only after an Exact answer can lose the join case that test_sorted_scan_removes_sort_from_sort_merge_join checks, because RepartitionExec::try_pushdown_sort returns Unsupported when the repartition doesn't keep its input's order. Measured at target_partitions = 4 with #55: an unfiltered sort-merge join keeps both scans' order and needs no SortExec, because the hash repartition over a single-partition scan keeps its input's order. With a WHERE on a join side, DataFusion adds a FilterExec and a round-robin repartition under the hash repartition, which loses the order. That side gets a SortExec while its scan still merges. The results are correct, but the merge is wasted, and pushdown can't remove it. Sorted aggregation never gets a SortExec, so pushdown can't help it either.

Options

1. Report no order, implement try_pushdown_sort, and merge only after an Exact answer. This fixes every case above, but loses filtered ORDER BY, sorted aggregation, and filtered or multi-partition join inputs.
2. Keep reporting the order, and add cheap paths:
   (a) when the sort above shares no leading column with the files' order, answer Inexact with a source that reads the files without merging. This is safe because the sort stays, and it fixes DESC and nullable ORDER BY id.
   (b) read files back to back instead of merging when their key ranges don't overlap (Bound the sorted-scan merge by how many files' key ranges overlap #71).
   Neither helps a SELECT * with no sort.
3. Ship a PhysicalOptimizerRule that turns merging off on scans whose order nothing relies on. It must run before OutputRequirements removal, so users would have to rebuild the rule list, which is new public API.
4. Change DataFusion: try_pushdown_sort on FilterExec, or a general "order is consumed" signal.

Proposal. Do 2(a) now, 2(b) once iceberg-rust has per-file bounds, and choose between 1 and 3 from measured plans. Feedback on that choice is very welcome.

I'm working on this. I have 2(a) implemented and will open it as a PR stacked on #55, and I'll drive the discussion for the rest.