[multistage][bugfix] fix literal query return multiple duplicate results

pinot-query-planner/src/test/resources/queries/LiteralEvaluationPlans.json

@@ -1,6 +1,15 @@
 {
   "literal_evaluation_tests": {
     "queries": [
+      {
+        "description": "Select without from",
+        "sql": "EXPLAIN PLAN FOR SELECT 1",
+        "output": [
+          "Execution Plan",
+          "\nLogicalValues(tuples=[[{ 1 }]])",
+          "\n"
+        ]
+      },
       {
         "description": "Select arithmetic expression",
         "sql": "EXPLAIN PLAN FOR SELECT 5*6,5+6 FROM d",

pinot-query-runtime/src/main/java/org/apache/pinot/query/runtime/operator/LiteralValueOperator.java

@@ -45,7 +45,8 @@ public LiteralValueOperator(OpChainExecutionContext context, DataSchema dataSche
     super(context);
     _dataSchema = dataSchema;
     _rexLiteralBlock = constructBlock(rexLiteralRows);
-    _isLiteralBlockReturned = false;
+    // only return a single literal block when it is the 1st virtual server. otherwise, result will be duplicated.
+    _isLiteralBlockReturned = context.getId().getVirtualServerId() != 0;
   }
 
   @Override

pinot-query-runtime/src/test/resources/queries/BasicQuery.json

@@ -16,6 +16,10 @@
       {
         "description": "basic test case example",
         "sql": "SELECT * FROM {tbl}"
+      },
+      {
+        "description": "basic test with literal",
+        "sql": "SELECT 1 AS int, CAST(2 AS DOUBLE) AS double"
       }
     ]
   },