Function pre_merge_scan_optimizer 

fn pre_merge_scan_optimizer() -> Optimizer

Builds the small optimizer pre-pass that runs before MergeScan wrapping.

This is intentionally not DataFusion’s full optimizer. After PlanRewriter wraps remote table scans in MergeScan, MergeScanLogicalPlan::inputs() hides remote_input, so ordinary optimizer rules can no longer see into the remote side. The main rule we need here is PushDownFilter: it moves side-local join/filter predicates into TableScan.filters, where region pruning and scan-level pruning can use them.

The rules before PushDownFilter are only the minimum cleanup/rewrite steps needed to make that filter pushdown safe around subqueries and set comparisons. For example, RewriteSetComparison handles ANY/ALL before they can become scan filters, and the decorrelation/subquery rules expose supported predicates as joins/filters instead of leaving raw subquery expressions under a scan.

Keep this list narrow. Do not add broad plan-shaping rules such as PushDownLimit, projection optimization, DISTINCT rewrites, or join-type rewrites here: those can change the local/remote distributed boundary or degrade unrelated planning diagnostics. Such rules belong either before this analyzer or after distributed planning, not in this pre-MergeScan, filter-focused pass.