[fix](nereids)AdjustNullable rule should handle union node with no children

fe/fe-core/src/main/java/org/apache/doris/nereids/rules/rewrite/AdjustNullable.java

@@ -142,45 +142,55 @@ public Plan visitLogicalRepeat(LogicalRepeat<? extends Plan> repeat, Map<ExprId,
     @Override
     public Plan visitLogicalSetOperation(LogicalSetOperation setOperation, Map<ExprId, Slot> replaceMap) {
         setOperation = (LogicalSetOperation) super.visit(setOperation, replaceMap);
-        if (setOperation.children().isEmpty()) {
-            return setOperation;
-        }
-        List<Boolean> inputNullable = setOperation.child(0).getOutput().stream()
-                .map(ExpressionTrait::nullable).collect(Collectors.toList());
         ImmutableList.Builder<List<SlotReference>> newChildrenOutputs = ImmutableList.builder();
-        for (int i = 0; i < setOperation.arity(); i++) {
-            List<Slot> childOutput = setOperation.child(i).getOutput();
-            List<SlotReference> setChildOutput = setOperation.getRegularChildOutput(i);
-            ImmutableList.Builder<SlotReference> newChildOutputs = ImmutableList.builder();
-            for (int j = 0; j < setChildOutput.size(); j++) {
-                for (Slot slot : childOutput) {
-                    if (slot.getExprId().equals(setChildOutput.get(j).getExprId())) {
-                        inputNullable.set(j, slot.nullable() || inputNullable.get(j));
-                        newChildOutputs.add((SlotReference) slot);
-                        break;
+        List<Boolean> inputNullable = null;
+        if (!setOperation.children().isEmpty()) {
+            inputNullable = setOperation.child(0).getOutput().stream()
+                    .map(ExpressionTrait::nullable).collect(Collectors.toList());
+            for (int i = 0; i < setOperation.arity(); i++) {
+                List<Slot> childOutput = setOperation.child(i).getOutput();
+                List<SlotReference> setChildOutput = setOperation.getRegularChildOutput(i);
+                ImmutableList.Builder<SlotReference> newChildOutputs = ImmutableList.builder();
+                for (int j = 0; j < setChildOutput.size(); j++) {
+                    for (Slot slot : childOutput) {
+                        if (slot.getExprId().equals(setChildOutput.get(j).getExprId())) {
+                            inputNullable.set(j, slot.nullable() || inputNullable.get(j));
+                            newChildOutputs.add((SlotReference) slot);
+                            break;
+                        }
                     }
                 }
+                newChildrenOutputs.add(newChildOutputs.build());
             }
-            newChildrenOutputs.add(newChildOutputs.build());
         }
         if (setOperation instanceof LogicalUnion) {
             LogicalUnion logicalUnion = (LogicalUnion) setOperation;
+            if (!logicalUnion.getConstantExprsList().isEmpty() && setOperation.children().isEmpty()) {
+                int outputSize = logicalUnion.getConstantExprsList().get(0).size();
+                // create the inputNullable list and fill it with all FALSE values
+                inputNullable = Lists.newArrayListWithCapacity(outputSize);
+                for (int i = 0; i < outputSize; i++) {
+                    inputNullable.add(false);
+                }
+            }
             for (List<NamedExpression> constantExprs : logicalUnion.getConstantExprsList()) {
                 for (int j = 0; j < constantExprs.size(); j++) {
-                    if (constantExprs.get(j).nullable()) {
-                        inputNullable.set(j, true);
-                    }
+                    inputNullable.set(j, inputNullable.get(j) || constantExprs.get(j).nullable());
                 }
             }
         }
+        if (inputNullable == null) {
+            // this is a fail-safe
+            // means there is no children and having no getConstantExprsList
+            // no way to update the nullable flag, so just do nothing
+            return setOperation;
+        }
         List<NamedExpression> outputs = setOperation.getOutputs();
         List<NamedExpression> newOutputs = Lists.newArrayListWithCapacity(outputs.size());
         for (int i = 0; i < inputNullable.size(); i++) {
             NamedExpression ne = outputs.get(i);
             Slot slot = ne instanceof Alias ? (Slot) ((Alias) ne).child() : (Slot) ne;
-            if (inputNullable.get(i)) {
-                slot = slot.withNullable(true);
-            }
+            slot = slot.withNullable(inputNullable.get(i));
             newOutputs.add(ne instanceof Alias ? (NamedExpression) ne.withChildren(slot) : slot);
         }
         newOutputs.forEach(o -> replaceMap.put(o.getExprId(), o.toSlot()));

regression-test/suites/nereids_syntax_p0/adjust_nullable.groovy

@@ -95,5 +95,23 @@ suite("adjust_nullable") {
     sql """
         drop table if exists table_8_undef_undef;
     """
+
+    sql """
+        drop table if exists orders_2_x;
+    """
+
+    sql """CREATE TABLE `orders_2_x` (
+        `o_orderdate` DATE not NULL
+        ) ENGINE=OLAP
+        DUPLICATE KEY(`o_orderdate`)
+        DISTRIBUTED BY HASH(`o_orderdate`) BUCKETS 96
+        PROPERTIES (
+        "replication_allocation" = "tag.location.default: 1"
+        );"""
+
+    explain {
+            sql(" select cast('2023-10-17' as date ) from orders_2_x union all select '2023-10-17' from orders_2_x;")
+            notContains("nullable=true")
+    }
 }