fix: fail on WINDOW clause without matching GROUP BY (#5431)

A WINDOW clause is only valid if there is no matching GROUP BY. Before this change, if user submitted a query without a GROUP BY it would silently be ignored, which is poor UX. With this change an helpful error message is displayed.

Co-authored-by: Andy Coates <big-andy-coates@users.noreply.github.com>

ksqldb-engine/src/main/java/io/confluent/ksql/planner/LogicalPlanner.java

@@ -43,6 +43,7 @@
 import io.confluent.ksql.function.udf.AsValue;
 import io.confluent.ksql.name.ColumnName;
 import io.confluent.ksql.name.SourceName;
+import io.confluent.ksql.parser.NodeLocation;
 import io.confluent.ksql.parser.tree.AllColumns;
 import io.confluent.ksql.parser.tree.GroupBy;
 import io.confluent.ksql.parser.tree.PartitionBy;
@@ -131,6 +132,13 @@ public OutputNode buildPlan() {
     if (analysis.getGroupBy().isPresent()) {
       currentNode = buildAggregateNode(currentNode);
     } else {
+      if (analysis.getWindowExpression().isPresent()) {
+        final String loc = analysis.getWindowExpression().get()
+            .getLocation()
+            .map(NodeLocation::asPrefix)
+            .orElse("");
+        throw new KsqlException(loc + "WINDOW clause requires a GROUP BY clause.");
+      }
       currentNode = buildUserProjectNode(currentNode);
     }
 

ksqldb-functional-tests/src/test/resources/query-validation-tests/windowed-source.json

@@ -13,6 +13,17 @@
         "type": "io.confluent.ksql.util.KsqlException",
         "message": "KSQL does not support persistent queries on windowed tables."
       }
+    },
+    {
+      "name": "window without group by",
+      "statements": [
+        "CREATE STREAM INPUT (ID BIGINT KEY, NAME varchar, VALUE bigint) WITH (kafka_topic='test_topic', value_format='DELIMITED');",
+        "CREATE STREAM OUTPUT as SELECT * FROM INPUT WINDOW TUMBLING (SIZE 30 SECONDS);"
+      ],
+      "expectedException": {
+        "type": "io.confluent.ksql.util.KsqlException",
+        "message": "WINDOW clause requires a GROUP BY clause."
+      }
     }
   ]
 }