diff --git a/fe/src/main/java/org/apache/doris/qe/Coordinator.java b/fe/src/main/java/org/apache/doris/qe/Coordinator.java
index 19042a3e9e..9d6103239b 100644
--- a/fe/src/main/java/org/apache/doris/qe/Coordinator.java
+++ b/fe/src/main/java/org/apache/doris/qe/Coordinator.java
@@ -833,18 +833,21 @@ public class Coordinator {
             }
 
             PlanNode leftMostNode = findLeftmostNode(fragment.getPlanRoot());
-            // When fragment contains UnionNode, because  the fragment may has child
-            // and not all BE will receive the fragment, child fragment's dest must
-            // be BE that fragment's scannode locates,  avoid less data.
-            // chenhao added
-            boolean hasUnionNode = containsUnionNode(fragment.getPlanRoot());
 
-            if (!(leftMostNode instanceof ScanNode) && !hasUnionNode) {
+            /*
+             * Case A:
+             *      if the left most is ScanNode, which means there is no child fragment,
+             *      we should assign fragment instances on every scan node hosts.
+             * Case B:
+             *      if not, there should be exchange nodes to collect all data from child fragments(input fragments),
+             *      so we should assign fragment instances corresponding to the child fragments' host
+             */
+            if (!(leftMostNode instanceof ScanNode)) {
+                // (Case B)
                 // there is no leftmost scan; we assign the same hosts as those of our
                 // leftmost input fragment (so that a partitioned aggregation
                 // fragment runs on the hosts that provide the input data)
-                PlanFragmentId inputFragmentIdx =
-                        fragments.get(i).getChild(0).getFragmentId();
+                PlanFragmentId inputFragmentIdx = fragments.get(i).getChild(0).getFragmentId();
                 // AddAll() soft copy()
                 int exchangeInstances = -1;
                 if (ConnectContext.get() != null && ConnectContext.get().getSessionVariable() != null) {
@@ -893,7 +896,7 @@ public class Coordinator {
                     params.instanceExecParams.add(instanceParam);
                 }
             } else {
-                // normal fragment
+                // case A
                 Iterator iter = fragmentExecParamsMap.get(fragment.getFragmentId()).scanRangeAssignment.entrySet().iterator();
                 int parallelExecInstanceNum = fragment.getParallel_exec_num();
                 while (iter.hasNext()) {