Fix PhysicalPlanNodeVisitor test coverage.

Signed-off-by: currantw <[email protected]>

Author: currantw

core/src/test/java/org/opensearch/sql/planner/physical/PhysicalPlanNodeVisitorTest.java

@@ -17,6 +17,7 @@
 import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.dedupe;
 import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.eval;
 import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.filter;
+import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.flatten;
 import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.limit;
 import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.project;
 import static org.opensearch.sql.planner.physical.PhysicalPlanDSL.rareTopN;
@@ -129,6 +130,8 @@ public static Stream<Arguments> getPhysicalPlanForTest() {
 
     PhysicalPlan eval = eval(plan, Pair.of(ref, ref));
 
+    PhysicalPlan flatten = flatten(plan, ref);
+
     PhysicalPlan sort = sort(plan, Pair.of(SortOption.DEFAULT_ASC, ref));
 
     PhysicalPlan takeOrdered = takeOrdered(plan, 1, 1, Pair.of(SortOption.DEFAULT_ASC, ref));
@@ -161,6 +164,7 @@ public static Stream<Arguments> getPhysicalPlanForTest() {
         Arguments.of(window, "window"),
         Arguments.of(remove, "remove"),
         Arguments.of(eval, "eval"),
+        Arguments.of(flatten, "flatten"),
         Arguments.of(sort, "sort"),
         Arguments.of(takeOrdered, "takeOrdered"),
         Arguments.of(dedupe, "dedupe"),