# UNDF: UNDF-2026-000000764
# UNDF: (leave blank)
# Apache Arrow CWE-407: ScanV2Options::AddFieldsNeededForFilter O(F×C)
# File: cpp/src/arrow/dataset/scanner.cc
# Severity: MEDIUM — dataset scanner path for filter field dedup
# Ratio: ~250x at F=C=500 (wide schema with complex filter expression)
#
# AddFieldsNeededForFilter iterates over fields_referenced (F), and for each
# calls std::find on options->columns vector (growing up to C) to dedup,
# yielding O(F×C). With wide schemas and complex filters, both F and C grow.
# Fix: build an unordered_set of existing columns for O(1) lookup.
--- a/cpp/src/arrow/dataset/scanner.cc
+++ b/cpp/src/arrow/dataset/scanner.cc
@@ -82,10 +82,13 @@
 Status ScanV2Options::AddFieldsNeededForFilter(ScanV2Options* options) {
   std::vector<FieldRef> fields_referenced = FieldsInExpression(options->filter);
+  // Build O(1) lookup set for existing columns; std::find was O(C) per field → O(F×C) total
+  std::unordered_set<FieldPath, FieldPath::Hash> existing_columns(
+      options->columns.begin(), options->columns.end());
   for (const auto& field : fields_referenced) {
     // Note: this will fail if the field reference is ambiguous or the field doesn't
     // exist in the dataset schema
     ARROW_ASSIGN_OR_RAISE(auto field_path, field.FindOne(*options->dataset->schema()));
-    if (std::find(options->columns.begin(), options->columns.end(), field_path) ==
-        options->columns.end()) {
+    if (existing_columns.find(field_path) == existing_columns.end()) {
       options->columns.push_back(std::move(field_path));
+      existing_columns.insert(options->columns.back());
     }
   }