diff --git a/source/libs/function/src/builtinsimpl.c b/source/libs/function/src/builtinsimpl.c
index 7081a65dd0f785924037e4c0a4acc2fb8d6ef698..77cde174eee2d063259337ebff380fb3b0c3dcc2 100644
--- a/source/libs/function/src/builtinsimpl.c
+++ b/source/libs/function/src/builtinsimpl.c
@@ -166,6 +166,7 @@ typedef struct SSampleInfo {
 
 typedef struct STailItem {
   int64_t timestamp;
+  bool    isNull;
   char    data[];
 } STailItem;
 
@@ -3124,7 +3125,7 @@ int32_t sampleFunction(SqlFunctionCtx* pCtx) {
 
   int32_t startOffset = pCtx->offset;
   for (int32_t i = pInput->startRowIndex; i < pInput->numOfRows + pInput->startRowIndex; i += 1) {
-    if (colDataIsNull_f(pInputCol->nullbitmap, i)) {
+    if (colDataIsNull_s(pInputCol, i)) {
       //colDataAppendNULL(pOutput, i);
       continue;
     }
@@ -3190,14 +3191,19 @@ bool tailFunctionSetup(SqlFunctionCtx *pCtx, SResultRowEntryInfo *pResultInfo) {
   size_t unitSize = sizeof(STailItem) + pInfo->colBytes;
   for (int32_t i = 0; i < pInfo->numOfPoints; ++i) {
     pInfo->pItems[i] = (STailItem *)(pItem + i * unitSize);
+    pInfo->pItems[i]->isNull = false;
   }
 
   return true;
 }
 
-static void tailAssignResult(STailItem* pItem, char *data, int32_t colBytes, TSKEY ts) {
+static void tailAssignResult(STailItem* pItem, char *data, int32_t colBytes, TSKEY ts, bool isNull) {
   pItem->timestamp = ts;
-  memcpy(pItem->data, data, colBytes);
+  if (isNull) {
+    pItem->isNull = true;
+  } else {
+    memcpy(pItem->data, data, colBytes);
+  }
 }
 
 static int32_t tailCompFn(const void *p1, const void *p2, const void *param) {
@@ -3206,14 +3212,14 @@ static int32_t tailCompFn(const void *p1, const void *p2, const void *param) {
   return compareInt64Val(&d1->timestamp, &d2->timestamp);
 }
 
-static void doTailAdd(STailInfo* pInfo, char *data, TSKEY ts) {
+static void doTailAdd(STailInfo* pInfo, char *data, TSKEY ts, bool isNull) {
   STailItem **pList = pInfo->pItems;
   if (pInfo->numAdded < pInfo->numOfPoints) {
-    tailAssignResult(pList[pInfo->numAdded], data, pInfo->colBytes, ts);
+    tailAssignResult(pList[pInfo->numAdded], data, pInfo->colBytes, ts, isNull);
     taosheapsort((void *)pList, sizeof(STailItem **), pInfo->numAdded + 1, NULL, tailCompFn, 0);
     pInfo->numAdded++;
   } else if (pList[0]->timestamp < ts) {
-    tailAssignResult(pList[0], data, pInfo->colBytes, ts);
+    tailAssignResult(pList[0], data, pInfo->colBytes, ts, isNull);
     taosheapadjust((void *)pList, sizeof(STailItem **), 0, pInfo->numOfPoints - 1, NULL, tailCompFn, NULL, 0);
   }
 }
@@ -3231,19 +3237,21 @@ int32_t tailFunction(SqlFunctionCtx* pCtx) {
 
   int32_t startOffset = pCtx->offset;
   for (int32_t i = pInput->startRowIndex; i < pInput->numOfRows + pInput->startRowIndex; i += 1) {
-    if (colDataIsNull_f(pInputCol->nullbitmap, i)) {
-      colDataAppendNULL(pOutput, i);
-      continue;
-    }
 
     char* data = colDataGetData(pInputCol, i);
-    doTailAdd(pInfo, data, tsList[i]);
+    doTailAdd(pInfo, data, tsList[i], colDataIsNull_s(pInputCol, i));
   }
 
+  taosqsort(pInfo->pItems, pInfo->numOfPoints, POINTER_BYTES, NULL, tailCompFn);
+
   for (int32_t i = 0; i < pInfo->numOfPoints; ++i) {
     int32_t pos = startOffset + i;
     STailItem *pItem = pInfo->pItems[i];
-    colDataAppend(pOutput, pos, pItem->data, false);
+    if (pItem->isNull) {
+      colDataAppendNULL(pOutput, pos);
+    } else {
+      colDataAppend(pOutput, pos, pItem->data, false);
+    }
   }
 
   return pInfo->numOfPoints;