tjbanghart commented on code in PR #4658:
URL: https://github.com/apache/calcite/pull/4658#discussion_r2586351008


##########
core/src/test/java/org/apache/calcite/test/enumerable/EnumerableCombineTest.java:
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@@ -0,0 +1,126 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to you under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.calcite.test.enumerable;
+
+import org.apache.calcite.adapter.java.ReflectiveSchema;
+import org.apache.calcite.config.CalciteConnectionProperty;
+import org.apache.calcite.config.Lex;
+import org.apache.calcite.test.CalciteAssert;
+import org.apache.calcite.test.schemata.hr.HrSchema;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link 
org.apache.calcite.adapter.enumerable.EnumerableCombine}.
+ */
+class EnumerableCombineTest {
+
+  /**
+   * Test that executes two simple queries combined.
+   * Query 1: Select employee names from department 10
+   * Query 2: Select department names
+   *
+   * <p>The Combine operator returns results in a structured format where each
+   * query's results are grouped: {@code QUERY_0={...}; QUERY_1={...}}
+   */
+  @Test void testCombineTwoQueries() {
+    tester(new HrSchema())
+        .withRel(
+            builder -> {
+              // Query 1: SELECT name FROM emps WHERE deptno = 10
+              builder.scan("s", "emps")
+                  .filter(
+                      builder.equals(
+                          builder.field("deptno"),
+                          builder.literal(10)))
+                  .project(builder.field("name"));
+
+              // Query 2: SELECT name FROM depts
+              builder.scan("s", "depts")
+                  .project(builder.field("name"));
+
+              // Combine both queries
+              return builder.combine(2).build();
+            })
+        .returnsUnordered(

Review Comment:
   To later accommodate named queries I've changed the format once more. From 
the comment in `EnumerableCombine`:
   
   
   The output format is a wide table where each column corresponds to a query 
(named QUERY_0, QUERY_1, etc.) and each row contains a struct with that query's 
column values for that row index. The number of output rows equals the maximum 
row count across all input queries. Queries with fewer rows have null values 
for the additional rows.
   
   Example output for two queries:
   ```
   QUERY_0                  | QUERY_1
   ------------------------ | ------------------------
   {empno=100, name=Bill}   | {deptno=10, name=Sales}
   {empno=110, name=Eric}   | {deptno=20, name=HR}
   {empno=120, name=Ted}    | null
   
   ```



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