diff --git a/src/client/src/tscFunctionImpl.c b/src/client/src/tscFunctionImpl.c
index 1ec84f023ad74cc8c29654bf14f55781485b691d..8e81462257fcd2b43dbbb32419a617ae2e8fa46f 100644
--- a/src/client/src/tscFunctionImpl.c
+++ b/src/client/src/tscFunctionImpl.c
@@ -1481,7 +1481,7 @@ static bool first_last_function_setup(SQLFunctionCtx *pCtx) {
 
 // todo opt for null block
 static void first_function(SQLFunctionCtx *pCtx) {
-  if (pCtx->order == TSDB_ORDER_DESC) {
+  if (pCtx->order == TSDB_ORDER_DESC || pCtx->preAggVals.dataBlockLoaded == false) {
     return;
   }
   
@@ -1550,28 +1550,17 @@ static void first_data_assign_impl(SQLFunctionCtx *pCtx, char *pData, int32_t in
  * to decide if the value is earlier than current intermediate result
  */
 static void first_dist_function(SQLFunctionCtx *pCtx) {
-  assert(pCtx->size > 0);
-
-  if (pCtx->size == 0) {
-    return;
-  }
-  
   /*
    * do not to check data in the following cases:
    * 1. data block that are not loaded
    * 2. scan data files in desc order
    */
-  if (pCtx->order == TSDB_ORDER_DESC) {
+  if (pCtx->order == TSDB_ORDER_DESC || pCtx->preAggVals.dataBlockLoaded == false) {
     return;
   }
   
   int32_t notNullElems = 0;
 
-  // data block is discard, not loaded, do not need to check it
-  if (!pCtx->preAggVals.dataBlockLoaded) {
-    return;
-  }
-
   // find the first not null value
   for (int32_t i = 0; i < pCtx->size; ++i) {
     char *data = GET_INPUT_CHAR_INDEX(pCtx, i);
@@ -1655,7 +1644,7 @@ static void first_dist_func_second_merge(SQLFunctionCtx *pCtx) {
  *    least one data in this block that is not null.(TODO opt for this case)
  */
 static void last_function(SQLFunctionCtx *pCtx) {
-  if (pCtx->order != pCtx->param[0].i64Key) {
+  if (pCtx->order != pCtx->param[0].i64Key || pCtx->preAggVals.dataBlockLoaded == false) {
     return;
   }
   
diff --git a/src/query/inc/qresultBuf.h b/src/query/inc/qresultBuf.h
index ad01555c2867b02b8365b50335930b81debd7130..a323d530a27ada5eabf9c4316281ee01d782c090 100644
--- a/src/query/inc/qresultBuf.h
+++ b/src/query/inc/qresultBuf.h
@@ -22,26 +22,22 @@ extern "C" {
 
 #include "os.h"
 #include "qextbuffer.h"
+#include "hash.h"
 
-typedef struct SIDList {
-  uint32_t alloc;
-  int32_t  size;
-  int32_t* pData;
-} SIDList;
+typedef struct SArray* SIDList;
 
 typedef struct SDiskbasedResultBuf {
-  int32_t  numOfRowsPerPage;
-  int32_t  numOfPages;
-  int64_t  totalBufSize;
-  int32_t  fd;                  // data file fd
-  int32_t  allocateId;          // allocated page id
-  int32_t  incStep;             // minimum allocated pages
-  char*    pBuf;                // mmap buffer pointer
-  char*    path;                // file path
+  int32_t   numOfRowsPerPage;
+  int32_t   numOfPages;
+  int64_t   totalBufSize;
+  int32_t   fd;                  // data file fd
+  int32_t   allocateId;          // allocated page id
+  int32_t   incStep;             // minimum allocated pages
+  char*     pBuf;                // mmap buffer pointer
+  char*     path;                // file path
   
-  uint32_t numOfAllocGroupIds;  // number of allocated id list
-  void*    idsTable;            // id hash table
-  SIDList* list;                // for each id, there is a page id list
+  SHashObj* idsTable;            // id hash table
+  SIDList   list;                // for each id, there is a page id list
 } SDiskbasedResultBuf;
 
 #define DEFAULT_INTERN_BUF_PAGE_SIZE (8192L*5)
@@ -112,7 +108,7 @@ void destroyResultBuf(SDiskbasedResultBuf* pResultBuf, void* handle);
  * @param pList
  * @return
  */
-int32_t getLastPageId(SIDList *pList);
+int32_t getLastPageId(SIDList pList);
 
 #ifdef __cplusplus
 }
diff --git a/src/query/src/qExecutor.c b/src/query/src/qExecutor.c
index 82cd4f4286bb445964bb15c5ad6ee01f92370732..d1349bd221ba18684097b25ad48b30f57d879c15 100644
--- a/src/query/src/qExecutor.c
+++ b/src/query/src/qExecutor.c
@@ -361,7 +361,7 @@ static bool hasTagValOutput(SQuery* pQuery) {
  * @return
  */
 static bool hasNullValue(SColIndex* pColIndex, SDataStatis *pStatis, SDataStatis **pColStatis) {
-  if (pStatis != NULL) {
+  if (pStatis != NULL && !TSDB_COL_IS_TAG(pColIndex->flag)) {
     *pColStatis = &pStatis[pColIndex->colIndex];
     assert((*pColStatis)->colId == pColIndex->colId);
   } else {
@@ -472,10 +472,10 @@ static int32_t addNewWindowResultBuf(SWindowResult *pWindowRes, SDiskbasedResult
   int32_t pageId = -1;
   SIDList list = getDataBufPagesIdList(pResultBuf, sid);
 
-  if (list.size == 0) {
+  if (taosArrayGetSize(list) == 0) {
     pData = getNewDataBuf(pResultBuf, sid, &pageId);
   } else {
-    pageId = getLastPageId(&list);
+    pageId = getLastPageId(list);
     pData = GET_RES_BUF_PAGE_BY_ID(pResultBuf, pageId);
 
     if (pData->num >= numOfRowsPerPage) {
@@ -2069,7 +2069,7 @@ int32_t loadDataBlockOnDemand(SQueryRuntimeEnv *pRuntimeEnv, void* pQueryHandle,
         int32_t colId = pSqlFunc->colInfo.colId;
 
         status |= aAggs[functionId].dataReqFunc(&pRuntimeEnv->pCtx[i], pBlockInfo->window.skey, pBlockInfo->window.ekey, colId);
-        if ((status & BLK_DATA_ALL_NEEDED) != 0) {
+        if ((status & BLK_DATA_ALL_NEEDED) == BLK_DATA_ALL_NEEDED) {
           break;
         }
       }
@@ -2670,16 +2670,19 @@ void copyResToQueryResultBuf(SQInfo *pQInfo, SQuery *pQuery) {
   SIDList list = getDataBufPagesIdList(pResultBuf, pQInfo->offset + id);
 
   int32_t total = 0;
-  for (int32_t i = 0; i < list.size; ++i) {
-    tFilePage *pData = GET_RES_BUF_PAGE_BY_ID(pResultBuf, list.pData[i]);
+  int32_t size = taosArrayGetSize(list);
+  for (int32_t i = 0; i < size; ++i) {
+    int32_t* pgId = taosArrayGet(list, i);
+    tFilePage *pData = GET_RES_BUF_PAGE_BY_ID(pResultBuf, *pgId);
     total += pData->num;
   }
 
   int32_t rows = total;
 
   int32_t offset = 0;
-  for (int32_t num = 0; num < list.size; ++num) {
-    tFilePage *pData = GET_RES_BUF_PAGE_BY_ID(pResultBuf, list.pData[num]);
+  for (int32_t j = 0; j < size; ++j) {
+    int32_t* pgId = taosArrayGet(list, j);
+    tFilePage *pData = GET_RES_BUF_PAGE_BY_ID(pResultBuf, *pgId);
 
     for (int32_t i = 0; i < pQuery->numOfOutput; ++i) {
       int32_t bytes = pRuntimeEnv->pCtx[i].outputBytes;
@@ -2745,7 +2748,7 @@ int32_t mergeIntoGroupResultImpl(SQInfo *pQInfo, SArray *pGroup) {
     STableQueryInfo *item = taosArrayGetP(pGroup, i);
 
     SIDList list = getDataBufPagesIdList(pRuntimeEnv->pResultBuf, TSDB_TABLEID(item->pTable)->tid);
-    if (list.size > 0 && item->windowResInfo.size > 0) {
+    if (taosArrayGetSize(list) > 0 && item->windowResInfo.size > 0) {
       pTableList[numOfTables] = item;
       numOfTables += 1;
     }
@@ -4208,14 +4211,14 @@ int32_t doInitQInfo(SQInfo *pQInfo, STSBuf *pTsBuf, void *tsdb, int32_t vgId, bo
 
   pRuntimeEnv->numOfRowsPerPage = getNumOfRowsInResultPage(pQuery, pRuntimeEnv->topBotQuery, isSTableQuery);
 
-  if (isSTableQuery) {
+  if (isSTableQuery && !onlyQueryTags(pRuntimeEnv->pQuery)) {
     int32_t rows = getInitialPageNum(pQInfo);
     code = createDiskbasedResultBuffer(&pRuntimeEnv->pResultBuf, rows, pQuery->rowSize, pQInfo);
     if (code != TSDB_CODE_SUCCESS) {
       return code;
     }
 
-    if (pQuery->intervalTime == 0) {
+    if (!QUERY_IS_INTERVAL_QUERY(pQuery)) {
       int16_t type = TSDB_DATA_TYPE_NULL;
 
       if (pRuntimeEnv->groupbyNormalCol) {  // group by columns not tags;
diff --git a/src/query/src/qresultBuf.c b/src/query/src/qresultBuf.c
index fe079694f7edd5e631b736c3150140436c9bca00..6175008ef663123bc631b06ce45bb4094084c27a 100644
--- a/src/query/src/qresultBuf.c
+++ b/src/query/src/qresultBuf.c
@@ -2,7 +2,6 @@
 #include "hash.h"
 #include "qextbuffer.h"
 #include "taoserror.h"
-#include "tsqlfunction.h"
 #include "queryLog.h"
 
 int32_t createDiskbasedResultBuffer(SDiskbasedResultBuf** pResultBuf, int32_t size, int32_t rowSize, void* handle) {
@@ -20,11 +19,10 @@ int32_t createDiskbasedResultBuffer(SDiskbasedResultBuf** pResultBuf, int32_t si
 
   // init id hash table
   pResBuf->idsTable = taosHashInit(size, taosGetDefaultHashFunction(TSDB_DATA_TYPE_INT), false);
-  pResBuf->list = calloc(size, sizeof(SIDList));
-  pResBuf->numOfAllocGroupIds = size;
+  pResBuf->list = taosArrayInit(size, POINTER_BYTES);
 
   char path[4096] = {0};
-  getTmpfilePath("tsdb_q_buf", path);
+  getTmpfilePath("tsdb_qbuf", path);
   pResBuf->path = strdup(path);
 
   pResBuf->fd = open(pResBuf->path, O_CREAT | O_RDWR, 0666);
@@ -48,7 +46,7 @@ int32_t createDiskbasedResultBuffer(SDiskbasedResultBuf** pResultBuf, int32_t si
     return TSDB_CODE_QRY_OUT_OF_MEMORY; // todo change error code
   }
 
-  qDebug("QInfo:%p create tmp file for output result, %s, %" PRId64 "bytes", handle, pResBuf->path,
+  qDebug("QInfo:%p create tmp file for output result: %s, %" PRId64 "bytes", handle, pResBuf->path,
       pResBuf->totalBufSize);
   
   return TSDB_CODE_SUCCESS;
@@ -90,7 +88,7 @@ static FORCE_INLINE bool noMoreAvailablePages(SDiskbasedResultBuf* pResultBuf) {
   return (pResultBuf->allocateId == pResultBuf->numOfPages - 1);
 }
 
-static int32_t getGroupIndex(SDiskbasedResultBuf* pResultBuf, int32_t groupId) {
+static FORCE_INLINE int32_t getGroupIndex(SDiskbasedResultBuf* pResultBuf, int32_t groupId) {
   assert(pResultBuf != NULL);
 
   char* p = taosHashGet(pResultBuf->idsTable, (const char*)&groupId, sizeof(int32_t));
@@ -99,51 +97,20 @@ static int32_t getGroupIndex(SDiskbasedResultBuf* pResultBuf, int32_t groupId) {
   }
 
   int32_t slot = GET_INT32_VAL(p);
-  assert(slot >= 0 && slot < pResultBuf->numOfAllocGroupIds);
+  assert(slot >= 0 && slot < taosHashGetSize(pResultBuf->idsTable));
 
   return slot;
 }
 
 static int32_t addNewGroupId(SDiskbasedResultBuf* pResultBuf, int32_t groupId) {
   int32_t num = getNumOfResultBufGroupId(pResultBuf); // the num is the newest allocated group id slot
-
-  if (pResultBuf->numOfAllocGroupIds <= num) {
-    size_t n = pResultBuf->numOfAllocGroupIds << 1u;
-
-    SIDList* p = (SIDList*)realloc(pResultBuf->list, sizeof(SIDList) * n);
-    assert(p != NULL);
-
-    memset(&p[pResultBuf->numOfAllocGroupIds], 0, sizeof(SIDList) * pResultBuf->numOfAllocGroupIds);
-    
-    pResultBuf->list = p;
-    pResultBuf->numOfAllocGroupIds = n;
-  }
-
   taosHashPut(pResultBuf->idsTable, (const char*)&groupId, sizeof(int32_t), &num, sizeof(int32_t));
-  return num;
-}
 
-static int32_t doRegisterId(SIDList* pList, int32_t id) {
-  if (pList->size >= pList->alloc) {
-    int32_t s = 0;
-    if (pList->alloc == 0) {
-      s = 4;
-      assert(pList->pData == NULL);
-    } else {
-      s = pList->alloc << 1u;
-    }
-    
-    int32_t* c = realloc(pList->pData, s * sizeof(int32_t));
-    assert(c);
-    
-    memset(&c[pList->alloc], 0, sizeof(int32_t) * pList->alloc);
-
-    pList->pData = c;
-    pList->alloc = s;
-  }
+  SArray* pa = taosArrayInit(1, sizeof(int32_t));
+  taosArrayPush(pResultBuf->list, &pa);
 
-  pList->pData[pList->size++] = id;
-  return 0;
+  assert(taosArrayGetSize(pResultBuf->list) == taosHashGetSize(pResultBuf->idsTable));
+  return num;
 }
 
 static void registerPageId(SDiskbasedResultBuf* pResultBuf, int32_t groupId, int32_t pageId) {
@@ -152,8 +119,8 @@ static void registerPageId(SDiskbasedResultBuf* pResultBuf, int32_t groupId, int
     slot = addNewGroupId(pResultBuf, groupId);
   }
 
-  SIDList* pList = &pResultBuf->list[slot];
-  doRegisterId(pList, pageId);
+  SIDList pList = taosArrayGetP(pResultBuf->list, slot);
+  taosArrayPush(pList, &pageId);
 }
 
 tFilePage* getNewDataBuf(SDiskbasedResultBuf* pResultBuf, int32_t groupId, int32_t* pageId) {
@@ -178,12 +145,11 @@ tFilePage* getNewDataBuf(SDiskbasedResultBuf* pResultBuf, int32_t groupId, int32
 int32_t getNumOfRowsPerPage(SDiskbasedResultBuf* pResultBuf) { return pResultBuf->numOfRowsPerPage; }
 
 SIDList getDataBufPagesIdList(SDiskbasedResultBuf* pResultBuf, int32_t groupId) {
-  SIDList list = {0};
   int32_t slot = getGroupIndex(pResultBuf, groupId);
   if (slot < 0) {
-    return list;
+    return taosArrayInit(1, sizeof(int32_t));
   } else {
-    return pResultBuf->list[slot];
+    return taosArrayGetP(pResultBuf->list, slot);
   }
 }
 
@@ -202,22 +168,20 @@ void destroyResultBuf(SDiskbasedResultBuf* pResultBuf, void* handle) {
   
   tfree(pResultBuf->path);
 
-  for (int32_t i = 0; i < pResultBuf->numOfAllocGroupIds; ++i) {
-    SIDList* pList = &pResultBuf->list[i];
-    tfree(pList->pData);
+  size_t size = taosArrayGetSize(pResultBuf->list);
+  for (int32_t i = 0; i < size; ++i) {
+    SArray* pa = taosArrayGetP(pResultBuf->list, i);
+    taosArrayDestroy(pa);
   }
 
-  tfree(pResultBuf->list);
+  taosArrayDestroy(pResultBuf->list);
   taosHashCleanup(pResultBuf->idsTable);
   
   tfree(pResultBuf);
 }
 
-int32_t getLastPageId(SIDList *pList) {
-  if (pList == NULL || pList->size <= 0) {
-    return -1;
-  }
-  
-  return pList->pData[pList->size - 1];
+int32_t getLastPageId(SIDList pList) {
+  size_t size = taosArrayGetSize(pList);
+  return *(int32_t*) taosArrayGet(pList, size - 1);
 }
 
diff --git a/src/tsdb/src/tsdbRead.c b/src/tsdb/src/tsdbRead.c
index 2fca34cd0acc23745b3bf93b4b6d8a8c4c5b3aff..fee8784b88183b86c2a3a417e697f802f63dcad8 100644
--- a/src/tsdb/src/tsdbRead.c
+++ b/src/tsdb/src/tsdbRead.c
@@ -90,6 +90,12 @@ typedef struct SBlockOrderSupporter {
   int32_t*            numOfBlocksPerTable;
 } SBlockOrderSupporter;
 
+typedef struct SIOCostSummary {
+  int64_t blockLoadTime;
+  int64_t statisInfoLoadTime;
+  int64_t blockMergeTime;
+} SIOCostSummary;
+
 typedef struct STsdbQueryHandle {
   STsdbRepo*     pTsdb;
   SQueryFilePos  cur;              // current position
@@ -116,6 +122,8 @@ typedef struct STsdbQueryHandle {
   SArray*        defaultLoadColumn;// default load column
   SDataBlockLoadInfo dataBlockLoadInfo; /* record current block load information */
   SLoadCompBlockInfo compBlockLoadInfo; /* record current compblock information in SQuery */
+
+  SIOCostSummary cost;
 } STsdbQueryHandle;
 
 static void changeQueryHandleForLastrowQuery(TsdbQueryHandleT pqHandle);
@@ -622,6 +630,8 @@ static bool doLoadFileDataBlock(STsdbQueryHandle* pQueryHandle, SCompBlock* pBlo
   tfree(data);
 
   int64_t et = taosGetTimestampUs() - st;
+
+  pQueryHandle->cost.blockLoadTime += et;
   tsdbDebug("%p load file block into buffer, elapsed time:%"PRId64 " us", pQueryHandle, et);
 
   return blockLoaded;
@@ -1784,23 +1794,22 @@ void tsdbRetrieveDataBlockInfo(TsdbQueryHandleT* pQueryHandle, SDataBlockInfo* p
 int32_t tsdbRetrieveDataBlockStatisInfo(TsdbQueryHandleT* pQueryHandle, SDataStatis** pBlockStatis) {
   STsdbQueryHandle* pHandle = (STsdbQueryHandle*) pQueryHandle;
   
-  SQueryFilePos* cur = &pHandle->cur;
-  if (cur->mixBlock) {
+  SQueryFilePos* c = &pHandle->cur;
+  if (c->mixBlock) {
     *pBlockStatis = NULL;
     return TSDB_CODE_SUCCESS;
   }
   
-  assert((cur->slot >= 0 && cur->slot < pHandle->numOfBlocks) ||
-      ((cur->slot == pHandle->numOfBlocks) && (cur->slot == 0)));
-  
-  STableBlockInfo* pBlockInfo = &pHandle->pDataBlockInfo[cur->slot];
-  
-  // file block with subblocks has no statistics data
+  STableBlockInfo* pBlockInfo = &pHandle->pDataBlockInfo[c->slot];
+  assert((c->slot >= 0 && c->slot < pHandle->numOfBlocks) || ((c->slot == pHandle->numOfBlocks) && (c->slot == 0)));
+
+  // file block with sub-blocks has no statistics data
   if (pBlockInfo->compBlock->numOfSubBlocks > 1) {
     *pBlockStatis = NULL;
     return TSDB_CODE_SUCCESS;
   }
-  
+
+  int64_t stime = taosGetTimestampUs();
   tsdbLoadCompData(&pHandle->rhelper, pBlockInfo->compBlock, NULL);
 
   // todo opt perf
@@ -1830,7 +1839,10 @@ int32_t tsdbRetrieveDataBlockStatisInfo(TsdbQueryHandleT* pQueryHandle, SDataSta
       pHandle->statis[i].max = pBlockInfo->compBlock->keyLast;
     }
   }
-  
+
+  int64_t elapsed = taosGetTimestampUs() - stime;
+  pHandle->cost.statisInfoLoadTime += elapsed;
+
   return TSDB_CODE_SUCCESS;
 }
 
@@ -2351,6 +2363,10 @@ void tsdbCleanupQueryHandle(TsdbQueryHandleT queryHandle) {
   tsdbUnRefMemTable(pQueryHandle->pTsdb, pQueryHandle->imem);
 
   tsdbDestroyHelper(&pQueryHandle->rhelper);
+
+  tsdbDebug(":io-cost summary: statis-info time:%"PRId64"us, datablock time:%" PRId64"us ,%p", pQueryHandle->cost.statisInfoLoadTime,
+      pQueryHandle->cost.blockLoadTime, pQueryHandle->qinfo);
+
   tfree(pQueryHandle);
 }