fix: skip WindowTopN rewrite when fetch is 0 to avoid PartitionedTopK panic

datafusion/physical-optimizer/src/window_topn.rs

@@ -141,6 +141,15 @@ impl WindowTopN {
         // Step 2: Extract limit from predicate (rn <= K, rn < K, etc.)
         let (col_idx, limit_n) = extract_window_limit(filter.predicate())?;
 
+        // A predicate such as `rn < 1` (or the flipped `1 > rn`) yields a fetch of
+        // 0. `ROW_NUMBER`/`RANK` are always >= 1, so no row can satisfy it and the
+        // correct result is empty. `PartitionedTopKExec` requires `k > 0` and would
+        // panic on `k = 0`, so bail out here and let the regular `FilterExec` produce
+        // the (empty) result instead of rewriting.
+        if limit_n == 0 {
+            return None;
+        }
+
         // Step 3: Walk through optional ProjectionExec and RepartitionExec to find BoundedWindowAggExec
         let child = filter.input();
         let (window_exec, intermediates) = find_window_below(child)?;

datafusion/sqllogictest/test_files/window_topn.slt

@@ -84,6 +84,29 @@ SELECT id, pk, val FROM (
 8 3 100
 9 3 50
 
+# Test 3b: rn < 1 has fetch = 0. ROW_NUMBER is always >= 1 so no row qualifies and
+# the result must be empty. The rewrite must not build a PartitionedTopKExec with k = 0
+# (which panics); it falls back to the regular filter. Regression for the k = 0 panic.
+query III rowsort
+SELECT id, pk, val FROM (
+  SELECT *, ROW_NUMBER() OVER (PARTITION BY pk ORDER BY val) as rn FROM window_topn_t
+) WHERE rn < 1;
+----
+
+# Test 3c: flipped form `1 > rn` (fetch = 0) — same empty result, no panic.
+query III rowsort
+SELECT id, pk, val FROM (
+  SELECT *, ROW_NUMBER() OVER (PARTITION BY pk ORDER BY val) as rn FROM window_topn_t
+) WHERE 1 > rn;
+----
+
+# Test 3d: rn <= 0 (fetch = 0) — empty, no panic.
+query III rowsort
+SELECT id, pk, val FROM (
+  SELECT *, ROW_NUMBER() OVER (PARTITION BY pk ORDER BY val) as rn FROM window_topn_t
+) WHERE rn <= 0;
+----
+
 # Test 4: Without PARTITION BY — should NOT optimize
 query II rowsort
 SELECT id, val FROM (