Preserve sort.getCount() limit pushdown contract in pushDownSort

The base OpenSearchIndexScanQueryBuilder.pushDownSort() pushes
sort.getCount() as a limit when non-zero. Our override validated
_score DESC and returned true, but did not preserve this contract.

SQL always sets count=0, so this was not reachable today, but PPL
or future callers may set a non-zero count to combine sort+limit
in one LogicalSort node. Preserve the behavior defensively.

Add focused test: LogicalSort(count=7) with _score DESC verifies
the count is pushed down as request size.

Signed-off-by: Eric Wei <mengwei.eric@gmail.com>

Author: mengweieric

opensearch/src/main/java/org/opensearch/sql/opensearch/storage/scan/VectorSearchQueryBuilder.java

@@ -86,7 +86,12 @@ public boolean pushDownSort(LogicalSort sort) {
             "vectorSearch only supports ORDER BY _score DESC; _score ASC is not supported");
       }
     }
-    // _score DESC is the natural knn order — accept without pushing sort to OpenSearch
+    // _score DESC is the natural knn order — no need to push the sort itself to OpenSearch.
+    // Preserve the parent's sort.getCount() → limit pushdown contract: SQL always sets count=0,
+    // but PPL or future callers may set a non-zero count to combine sort+limit in one node.
+    if (sort.getCount() != 0) {
+      requestBuilder.pushDownLimit(sort.getCount(), 0);
+    }
     return true;
   }
 }

opensearch/src/test/java/org/opensearch/sql/opensearch/storage/scan/VectorSearchQueryBuilderTest.java

@@ -155,6 +155,31 @@ void pushDownSortScoreDescAccepted() {
     assertTrue(pushed, "ORDER BY _score DESC should be accepted");
   }
 
+  @Test
+  void pushDownSortPreservesSortCountAsLimit() {
+    var requestBuilder = createRequestBuilder();
+    var knnQuery = new WrapperQueryBuilder("{\"knn\":{}}");
+    var builder = new VectorSearchQueryBuilder(requestBuilder, knnQuery, Map.of("k", "10"));
+
+    var dummyChild = new LogicalValues(Collections.emptyList());
+    // LogicalSort with count=7 simulates a sort+limit combined node (PPL path)
+    var sort =
+        new org.opensearch.sql.planner.logical.LogicalSort(
+            dummyChild,
+            7,
+            List.of(
+                org.apache.commons.lang3.tuple.ImmutablePair.of(
+                    org.opensearch.sql.ast.tree.Sort.SortOption.DEFAULT_DESC,
+                    new ReferenceExpression("_score", ExprCoreType.FLOAT))));
+
+    boolean pushed = builder.pushDownSort(sort);
+    assertTrue(pushed, "ORDER BY _score DESC with count should be accepted");
+    assertEquals(
+        7,
+        requestBuilder.getMaxResponseSize(),
+        "sort.getCount() should be pushed down as request size");
+  }
+
   @Test
   void pushDownSortNonScoreFieldRejected() {
     var requestBuilder = createRequestBuilder();