Antalya 26.8: Do not reuse a server constant built in another scope.

src/Analyzer/Resolve/resolveFunction.cpp

@@ -2775,8 +2775,12 @@ ProjectionNames QueryAnalyzer::resolveFunction(QueryTreeNodePtr & node, Identifi
         /// `randConstant(), randConstant()` shares. What separates two calls is their arguments, which
         /// the hash still covers: `randConstant(1)` and `randConstant(2)` keep their own values, and that
         /// is the documented way to ask for two different constants in one query.
+        ///
+        /// Server constants (`hostName`, `serverUUID`, ...) capture `Context::isDistributed` when built.
+        /// A local `UNION` branch and a later cluster branch must not share that instance.
         if (function && !function->isDeterministic() && !function->isStateful()
-            && function->isDeterministicInScopeOfQuery())
+            && function->isDeterministicInScopeOfQuery()
+            && !function->isServerConstant())
         {
             auto hash = function_node_ptr->getTreeHash({ .compare_aliases = false });
             function_base_cache = &functions_cache[hash];

tests/queries/0_stateless/05294_union_hostname_with_cluster_function.reference

@@ -0,0 +1,2 @@
+1	1	1
+2	2	1

tests/queries/0_stateless/05294_union_hostname_with_cluster_function.sql

@@ -0,0 +1,15 @@
+-- A local GROUP BY hostName() must not be reused for a later cluster branch.
+-- hostName() is folded on the local branch and must stay a per-shard column on the cluster branch.
+
+SELECT tag, c, h = hostName()
+FROM
+(
+    SELECT hostName() AS h, count() AS c, 1 AS tag
+    FROM numbers(1)
+    GROUP BY h
+    UNION ALL
+    SELECT hostName() AS h, count() AS c, 2 AS tag
+    FROM cluster('test_cluster_two_shards', system.one)
+    GROUP BY h
+)
+ORDER BY tag;