diff --git a/source/libs/executor/src/scanoperator.c b/source/libs/executor/src/scanoperator.c
index 6480aa9104..3e8574f49f 100644
--- a/source/libs/executor/src/scanoperator.c
+++ b/source/libs/executor/src/scanoperator.c
@@ -3302,12 +3302,15 @@ static SSDataBlock* getBlockForTableMergeScan(void* param) {
   STsdbReader* reader = pInfo->base.dataReader;
   while (true) {
     if (pInfo->rtnNextDurationBlocks) {
+      qDebug("%s table merge scan return already fetched new duration blocks. index %d num of blocks %d", 
+              GET_TASKID(pTaskInfo), pInfo->nextDurationBlocksIdx, pInfo->numNextDurationBlocks);
       if (pInfo->nextDurationBlocksIdx < pInfo->numNextDurationBlocks) {
         pBlock = pInfo->nextDurationBlocks[pInfo->nextDurationBlocksIdx];
         ++pInfo->nextDurationBlocksIdx;
       } else {
         for (int32_t i = 0; i < pInfo->numNextDurationBlocks; ++i) {
           blockDataDestroy(pInfo->nextDurationBlocks[i]);
+          pInfo->nextDurationBlocks[i] = NULL;
         }
         pInfo->rtnNextDurationBlocks = false;
         pInfo->nextDurationBlocksIdx = 0;
@@ -3320,7 +3323,8 @@ static SSDataBlock* getBlockForTableMergeScan(void* param) {
       bool bSkipped = false;
       doGetBlockForTableMergeScan(pOperator, &bFinished, &bSkipped);
       pBlock = pInfo->pReaderBlock;
-
+      qDebug("%s table merge scan fetch block. finished %d skipped %d next-duration-block %d new-fileset %d", 
+              GET_TASKID(pTaskInfo), bFinished, bSkipped, pInfo->bNextDurationBlockEvent, pInfo->bNewFilesetEvent);
       if (bFinished) {
         pInfo->bNewFilesetEvent = false;
         break;
@@ -3330,7 +3334,11 @@ static SSDataBlock* getBlockForTableMergeScan(void* param) {
         if (!bSkipped) {
           pInfo->nextDurationBlocks[pInfo->numNextDurationBlocks] = createOneDataBlock(pBlock, true);
           ++pInfo->numNextDurationBlocks;
-          ASSERT(pInfo->numNextDurationBlocks <= 2);
+          if (pInfo->numNextDurationBlocks > 2) {
+            qError("%s table merge scan prefetch %d next duration blocks. end early.", GET_TASKID(pTaskInfo), pInfo->numNextDurationBlocks);
+            pInfo->bNewFilesetEvent = false;
+            break;
+          }
         }
         if (pInfo->bNewFilesetEvent) {
           pInfo->rtnNextDurationBlocks = true;
@@ -3394,6 +3402,8 @@ void tableMergeScanTsdbNotifyCb(ETsdReaderNotifyType type, STsdReaderNotifyInfo*
   } else if (type == TSD_READER_NOTIFY_NEXT_DURATION_BLOCK) {
     pTmsInfo->bNextDurationBlockEvent = true;
   }
+  qDebug("table merge scan receive notification. type %d, fileset %d", type, info->duration.filesetId);
+
   return;
 }
 
@@ -3403,6 +3413,7 @@ int32_t startDurationForGroupTableMergeScan(SOperatorInfo* pOperator) {
   int32_t code = TSDB_CODE_SUCCESS;
   int32_t numOfTable = pInfo->tableEndIndex - pInfo->tableStartIndex + 1;
 
+  qDebug("%s table merge scan start duration ", GET_TASKID(pTaskInfo));
   pInfo->bNewFilesetEvent = false;
   pInfo->bNextDurationBlockEvent = false;
 
@@ -3434,6 +3445,8 @@ int32_t startDurationForGroupTableMergeScan(SOperatorInfo* pOperator) {
 
 void stopDurationForGroupTableMergeScan(SOperatorInfo* pOperator) {
   STableMergeScanInfo* pInfo = pOperator->info;
+  SExecTaskInfo* pTaskInfo = pOperator->pTaskInfo;
+  qDebug("%s table merge scan stop duration ", GET_TASKID(pTaskInfo));
 
   SSortExecInfo sortExecInfo = tsortGetSortExecInfo(pInfo->pSortHandle);
   pInfo->sortExecInfo.sortMethod = sortExecInfo.sortMethod;
@@ -3451,6 +3464,7 @@ int32_t startGroupTableMergeScan(SOperatorInfo* pOperator) {
   SExecTaskInfo*       pTaskInfo = pOperator->pTaskInfo;
   SReadHandle* pHandle = &pInfo->base.readHandle;
   SStorageAPI* pAPI = &pTaskInfo->storageAPI;
+  qDebug("%s table merge scan start group %"PRIu64, GET_TASKID(pTaskInfo), pInfo->groupId);
 
   {
     size_t  numOfTables = tableListGetSize(pInfo->base.pTableListInfo);
@@ -3498,10 +3512,19 @@ int32_t stopGroupTableMergeScan(SOperatorInfo* pOperator) {
     pAPI->tsdReader.tsdReaderClose(pInfo->base.dataReader);
     pInfo->base.dataReader = NULL;
   }
-
+  for (int32_t i = 0; i < pInfo->numNextDurationBlocks; ++i) {
+    if (pInfo->nextDurationBlocks[i]) {
+      blockDataDestroy(pInfo->nextDurationBlocks[i]);
+      pInfo->nextDurationBlocks[i] = NULL;
+    }
+    pInfo->numNextDurationBlocks = 0;
+    pInfo->nextDurationBlocksIdx = 0;
+  }
   resetLimitInfoForNextGroup(&pInfo->limitInfo);
   taosHashCleanup(pInfo->mSkipTables);
   pInfo->mSkipTables = NULL;
+  qDebug("%s table merge scan stop group %"PRIu64, GET_TASKID(pTaskInfo), pInfo->groupId);
+
   return TSDB_CODE_SUCCESS;
 }
 
@@ -3612,6 +3635,13 @@ void destroyTableMergeScanOperatorInfo(void* param) {
   pTableScanInfo->base.readerAPI.tsdReaderClose(pTableScanInfo->base.dataReader);
   pTableScanInfo->base.dataReader = NULL;
 
+  for (int32_t i = 0; i < pTableScanInfo->numNextDurationBlocks; ++i) {
+    if (pTableScanInfo->nextDurationBlocks[i] != NULL) {
+      blockDataDestroy(pTableScanInfo->nextDurationBlocks[i]);
+      pTableScanInfo->nextDurationBlocks[i] = NULL;
+    }
+  }
+
   taosArrayDestroy(pTableScanInfo->sortSourceParams);
   tsortDestroySortHandle(pTableScanInfo->pSortHandle);
   pTableScanInfo->pSortHandle = NULL;