Merge pull request #3658 from taosdata/feature/query

Feature/query

src/client/inc/tscUtil.h

@@ -258,6 +258,8 @@ void tscDoQuery(SSqlObj* pSql);
  */
 SSqlObj* createSimpleSubObj(SSqlObj* pSql, void (*fp)(), void* param, int32_t cmd);
 
+void registerSqlObj(SSqlObj* pSql);
+
 SSqlObj* createSubqueryObj(SSqlObj* pSql, int16_t tableIndex, void (*fp)(), void* param, int32_t cmd, SSqlObj* pPrevSql);
 void     addGroupInfoForSubquery(SSqlObj* pParentObj, SSqlObj* pSql, int32_t subClauseIndex, int32_t tableIndex);
 

src/client/inc/tsclient.h

@@ -107,7 +107,7 @@ typedef struct STableMeta {
 } STableMeta;
 
 typedef struct STableMetaInfo {
-  STableMeta *  pTableMeta;      // table meta, cached in client side and acquired by name
+  STableMeta   *pTableMeta;      // table meta, cached in client side and acquired by name
   SVgroupsInfo *vgroupList;
   SArray       *pVgroupTables;   // SArray<SVgroupTableInfo>
   

src/client/src/tscAsync.c

@@ -51,10 +51,7 @@ void doAsyncQuery(STscObj* pObj, SSqlObj* pSql, void (*fp)(), void* param, const
   pSql->fp        = fp;
   pSql->fetchFp   = fp;
 
-  uint64_t handle = (uint64_t) pSql;
-  pSql->self = taosCachePut(tscObjCache, &handle, sizeof(uint64_t), &pSql, sizeof(uint64_t), 2*3600*1000);
-
-  T_REF_INC(pSql->pTscObj);
+  registerSqlObj(pSql);
 
   pSql->sqlstr = calloc(1, sqlLen + 1);
   if (pSql->sqlstr == NULL) {

src/client/src/tscPrepare.c

@@ -546,9 +546,7 @@ int taos_stmt_prepare(TAOS_STMT* stmt, const char* sql, unsigned long length) {
     pSql->cmd.numOfParams = 0;
     pSql->cmd.batchSize   = 0;
 
-    uint64_t handle = (uint64_t) pSql;
-    pSql->self = taosCachePut(tscObjCache, &handle, sizeof(uint64_t), &pSql, sizeof(uint64_t), 2*3600*1000);
-    T_REF_INC(pSql->pTscObj);
+    registerSqlObj(pSql);
 
     int32_t code = tsParseSql(pSql, true);
     if (code == TSDB_CODE_TSC_ACTION_IN_PROGRESS) {

src/client/src/tscServer.c

@@ -128,6 +128,7 @@ static void tscUpdateVgroupInfo(SSqlObj *pObj, SRpcEpSet *pEpSet) {
   tscDebug("after: EndPoint in use: %d", pVgroupInfo->inUse);
   taosCorEndWrite(&pVgroupInfo->version);
 }
+
 void tscPrintMgmtEp() {
   SRpcEpSet dump;
   tscDumpMgmtEpSet(&dump);
@@ -745,7 +746,6 @@ int tscBuildQueryMsg(SSqlObj *pSql, SSqlInfo *pInfo) {
     SSqlExpr *pExpr = tscSqlExprGet(pQueryInfo, i);
 
     if (!tscValidateColumnId(pTableMetaInfo, pExpr->colInfo.colId, pExpr->numOfParams)) {
-      /* column id is not valid according to the cached table meta, the table meta is expired */
       tscError("%p table schema is not matched with parsed sql", pSql);
       return TSDB_CODE_TSC_INVALID_SQL;
     }
@@ -1987,15 +1987,11 @@ static void createHBObj(STscObj* pObj) {
   pSql->param = pObj;
   pSql->pTscObj = pObj;
   pSql->signature = pSql;
-  pObj->pHb = pSql;
-
-  tscAddSubqueryInfo(&pObj->pHb->cmd);
 
-  int64_t ad = (int64_t) pSql;
-  pSql->self = taosCachePut(tscObjCache, &ad, sizeof(int64_t), &pSql, sizeof(int64_t), 2 * 60 * 1000);
-  T_REF_INC(pObj);
+  registerSqlObj(pSql);
+  tscDebug("%p HB is allocated, pObj:%p", pSql, pObj);
 
-  tscDebug("%p HB is allocated, pObj:%p", pObj->pHb, pObj);
+  pObj->pHb = pSql;
 }
 
 int tscProcessConnectRsp(SSqlObj *pSql) {
@@ -2170,11 +2166,7 @@ static int32_t getTableMetaFromMgmt(SSqlObj *pSql, STableMetaInfo *pTableMetaInf
   pNew->signature = pNew;
   pNew->cmd.command = TSDB_SQL_META;
 
-  T_REF_INC(pNew->pTscObj);
-
-  // TODO add test case on x86 platform
-  uint64_t adr = (uint64_t) pNew;
-  pNew->self = taosCachePut(tscObjCache, &adr, sizeof(uint64_t), &pNew, sizeof(uint64_t), 2*60*1000);
+  registerSqlObj(pNew);
 
   tscAddSubqueryInfo(&pNew->cmd);
 
@@ -2301,10 +2293,8 @@ int tscGetSTableVgroupInfo(SSqlObj *pSql, int32_t clauseIndex) {
   }
 
   pNewQueryInfo->numOfTables = pQueryInfo->numOfTables;
-  T_REF_INC(pNew->pTscObj);
+  registerSqlObj(pNew);
 
-  uint64_t p = (uint64_t) pNew;
-  pNew->self = taosCachePut(tscObjCache, &p, sizeof(uint64_t), &pNew, sizeof(uint64_t), 2 * 600 * 1000);
   tscDebug("%p new sqlObj:%p to get vgroupInfo, numOfTables:%d", pSql, pNew, pNewQueryInfo->numOfTables);
 
   pNew->fp = tscTableMetaCallBack;

src/client/src/tscSql.c

@@ -156,10 +156,7 @@ SSqlObj *taosConnectImpl(const char *ip, const char *user, const char *pass, con
     *taos = pObj;
   }
 
-  T_REF_INC(pSql->pTscObj);
-
-  uint64_t key = (uint64_t) pSql;
-  pSql->self = taosCachePut(tscObjCache, &key, sizeof(uint64_t), &pSql, sizeof(uint64_t), 2*3600*1000);
+  registerSqlObj(pSql);
   tsInsertHeadSize = sizeof(SMsgDesc) + sizeof(SSubmitMsg);
 
   return pSql;
@@ -270,7 +267,7 @@ void taos_close(TAOS *taos) {
       pHb->pRpcCtx = NULL;
     }
 
-    tscDebug("%p, HB is freed", pHb);
+    tscDebug("%p HB is freed", pHb);
     taos_free_result(pHb);
   }
 
@@ -789,14 +786,17 @@ int taos_validate_sql(TAOS *taos, const char *sql) {
   }
 
   SSqlObj* pSql = calloc(1, sizeof(SSqlObj));
+
   pSql->pTscObj = taos;
   pSql->signature = pSql;
+
   SSqlRes *pRes = &pSql->res;
   SSqlCmd *pCmd = &pSql->cmd;
   
   pRes->numOfTotal = 0;
   pRes->numOfClauseTotal = 0;
 
+
   tscDebug("%p Valid SQL: %s pObj:%p", pSql, sql, pObj);
 
   int32_t sqlLen = (int32_t)strlen(sql);
@@ -832,11 +832,12 @@ int taos_validate_sql(TAOS *taos, const char *sql) {
     tsem_wait(&pSql->rspSem);
     code = pSql->res.code;
   }
+
   if (code != TSDB_CODE_SUCCESS) {
     tscDebug("%p Valid SQL result:%d, %s pObj:%p", pSql, code, taos_errstr(taos), pObj);
   }
-  taos_free_result(pSql);
 
+  taos_free_result(pSql);
   return code;
 }
 
@@ -935,34 +936,32 @@ int taos_load_table_info(TAOS *taos, const char *tableNameList) {
   SSqlObj* pSql = calloc(1, sizeof(SSqlObj));
   pSql->pTscObj = taos;
   pSql->signature = pSql;
+
   SSqlRes *pRes = &pSql->res;
 
+  pRes->code = 0;
   pRes->numOfTotal = 0;  // the number of getting table meta from server
   pRes->numOfClauseTotal = 0;
 
-  pRes->code = 0;
-
   assert(pSql->fp == NULL);
   tscDebug("%p tableNameList: %s pObj:%p", pSql, tableNameList, pObj);
 
   int32_t tblListLen = (int32_t)strlen(tableNameList);
   if (tblListLen > MAX_TABLE_NAME_LENGTH) {
     tscError("%p tableNameList too long, length:%d, maximum allowed:%d", pSql, tblListLen, MAX_TABLE_NAME_LENGTH);
-    pRes->code = TSDB_CODE_TSC_INVALID_SQL;
-    taosTFree(pSql);
-    return pRes->code;
+    tscFreeSqlObj(pSql);
+    return TSDB_CODE_TSC_INVALID_SQL;
   }
 
   char *str = calloc(1, tblListLen + 1);
   if (str == NULL) {
-    pRes->code = TSDB_CODE_TSC_OUT_OF_MEMORY;
     tscError("%p failed to malloc sql string buffer", pSql);
-    taosTFree(pSql);
-    return pRes->code;
+    tscFreeSqlObj(pSql);
+    return TSDB_CODE_TSC_OUT_OF_MEMORY;
   }
 
   strtolower(str, tableNameList);
-  pRes->code = (uint8_t)tscParseTblNameList(pSql, str, tblListLen);
+  int32_t code = (uint8_t) tscParseTblNameList(pSql, str, tblListLen);
 
   /*
    * set the qhandle to 0 before return in order to erase the qhandle value assigned in the previous successful query.
@@ -972,17 +971,17 @@ int taos_load_table_info(TAOS *taos, const char *tableNameList) {
   pRes->qhandle = 0;
   free(str);
 
-  if (pRes->code != TSDB_CODE_SUCCESS) {
+  if (code != TSDB_CODE_SUCCESS) {
     tscFreeSqlObj(pSql);
-    return pRes->code;
+    return code;
   }
 
   tscDoQuery(pSql);
 
-  tscDebug("%p load multi metermeta result:%d %s pObj:%p", pSql, pRes->code, taos_errstr(taos), pObj);
-  if (pRes->code != TSDB_CODE_SUCCESS) {
-    tscPartiallyFreeSqlObj(pSql);
+  tscDebug("%p load multi table meta result:%d %s pObj:%p", pSql, pRes->code, taos_errstr(taos), pObj);
+  if ((code = pRes->code) != TSDB_CODE_SUCCESS) {
+    tscFreeSqlObj(pSql);
   }
 
-  return pRes->code;
+  return code;
 }

src/client/src/tscStream.c

@@ -510,9 +510,7 @@ static void tscCreateStream(void *param, TAOS_RES *res, int code) {
     return;
   }
 
-  uint64_t handle = (uint64_t) pSql;
-  pSql->self = taosCachePut(tscObjCache, &handle, sizeof(uint64_t), &pSql, sizeof(uint64_t), 2*3600*1000);
-  T_REF_INC(pSql->pTscObj);
+  registerSqlObj(pSql);
 
   SQueryInfo* pQueryInfo = tscGetQueryInfoDetail(pCmd, 0);
   STableMetaInfo* pTableMetaInfo = tscGetMetaInfo(pQueryInfo, 0);

src/client/src/tscSub.c

@@ -105,6 +105,7 @@ static SSub* tscCreateSubscription(STscObj* pObj, const char* topic, const char*
     code = TAOS_SYSTEM_ERROR(errno);
     goto fail;
   }
+
   tstrncpy(pSub->topic, topic, sizeof(pSub->topic));
   pSub->progress = taosArrayInit(32, sizeof(SSubscriptionProgress));
   if (pSub->progress == NULL) {
@@ -119,6 +120,7 @@ static SSub* tscCreateSubscription(STscObj* pObj, const char* topic, const char*
     code = TSDB_CODE_TSC_OUT_OF_MEMORY;
     goto fail;
   }
+
   pSql->signature = pSql;
   pSql->pTscObj = pObj;
   pSql->pSubscription = pSub;
@@ -142,6 +144,7 @@ static SSub* tscCreateSubscription(STscObj* pObj, const char* topic, const char*
     code = TSDB_CODE_TSC_OUT_OF_MEMORY;
     goto fail;
   }
+
   strtolower(pSql->sqlstr, pSql->sqlstr);
   pRes->qhandle = 0;
   pRes->numOfRows = 1;
@@ -152,15 +155,14 @@ static SSub* tscCreateSubscription(STscObj* pObj, const char* topic, const char*
     goto fail;
   }
 
-  uint64_t handle = (uint64_t) pSql;
-  pSql->self = taosCachePut(tscObjCache, &handle, sizeof(uint64_t), &pSql, sizeof(uint64_t), 2*3600*1000);
-  T_REF_INC(pSql->pTscObj);
+  registerSqlObj(pSql);
 
   code = tsParseSql(pSql, false);
   if (code == TSDB_CODE_TSC_ACTION_IN_PROGRESS) {
     tsem_wait(&pSub->sem);
     code = pSql->res.code;
   }
+
   if (code != TSDB_CODE_SUCCESS) {
     line = __LINE__;
     goto fail;
@@ -182,8 +184,10 @@ fail:
     } else {
       tscFreeSqlObj(pSql);
     }
+
     pSql = NULL;
   }
+
   if (pSub != NULL) {
     taosArrayDestroy(pSub->progress);
     tsem_destroy(&pSub->sem);

src/client/src/tscSystem.c

@@ -122,11 +122,8 @@ void taos_init_imp(void) {
   tscInitMsgsFp();
   int queueSize = tsMaxConnections*2;
 
-  if (tscEmbedded == 0) {
-    tscNumOfThreads = (int)(tsNumOfCores * tsNumOfThreadsPerCore / 2.0);
-  } else {
-    tscNumOfThreads = (int)(tsNumOfCores * tsNumOfThreadsPerCore / 4.0);
-  }
+  double factor = (tscEmbedded == 0)? 2.0:4.0;
+  tscNumOfThreads = (int)(tsNumOfCores * tsNumOfThreadsPerCore / factor);
 
   if (tscNumOfThreads < 2) tscNumOfThreads = 2;
 
@@ -141,10 +138,7 @@ void taos_init_imp(void) {
     taosTmrReset(tscCheckDiskUsage, 10, NULL, tscTmr, &tscCheckDiskUsageTmr);      
   }
 
-  int64_t refreshTime = tsTableMetaKeepTimer;
-  refreshTime = refreshTime > 10 ? 10 : refreshTime;
-  refreshTime = refreshTime < 10 ? 10 : refreshTime;
-
+  int64_t refreshTime = 10; // 10 seconds by default
   if (tscMetaCache == NULL) {
     tscMetaCache = taosCacheInit(TSDB_DATA_TYPE_BINARY, refreshTime, false, NULL, "tableMeta");
     tscObjCache = taosCacheInit(TSDB_DATA_TYPE_BIGINT, refreshTime/2, false, tscFreeSqlObjInCache, "sqlObj");

src/client/src/tscUtil.c

@@ -391,10 +391,21 @@ static UNUSED_FUNC void tscFreeSubobj(SSqlObj* pSql) {
  */
 void tscFreeSqlObjInCache(void *pSql) {
   assert(pSql != NULL);
+
   SSqlObj** p = (SSqlObj**)pSql;
+  STscObj* pTscObj = (*p)->pTscObj;
 
   assert((*p)->self != 0 && (*p)->self == (p));
   tscFreeSqlObj(*p);
+
+  int32_t ref = T_REF_DEC(pTscObj);
+  assert(ref >= 0);
+
+  tscDebug("%p free sqlObj completed, tscObj:%p ref:%d", *p, pTscObj, ref);
+  if (ref == 0) {
+    tscDebug("%p all sqlObj freed, free tscObj:%p", *p, pTscObj);
+    tscCloseTscObj(pTscObj);
+  }
 }
 
 void tscFreeSqlObj(SSqlObj* pSql) {
@@ -403,7 +414,6 @@ void tscFreeSqlObj(SSqlObj* pSql) {
   }
 
   tscDebug("%p start to free sqlObj", pSql);
-  STscObj* pTscObj = pSql->pTscObj;
 
   tscFreeSubobj(pSql);
   tscPartiallyFreeSqlObj(pSql);
@@ -421,14 +431,6 @@ void tscFreeSqlObj(SSqlObj* pSql) {
   tsem_destroy(&pSql->rspSem);
 
   free(pSql);
-  tscDebug("%p free sqlObj completed", pSql);
-
-  int32_t ref = T_REF_DEC(pTscObj);
-  assert(ref >= 0);
-
-  if (ref == 0) {
-    tscCloseTscObj(pTscObj);
-  }
 }
 
 void tscDestroyDataBlock(STableDataBlocks* pDataBlock) {
@@ -1780,6 +1782,16 @@ void tscResetForNextRetrieve(SSqlRes* pRes) {
   pRes->numOfRows = 0;
 }
 
+void registerSqlObj(SSqlObj* pSql) {
+  int32_t DEFAULT_LIFE_TIME = 2 * 600 * 1000;  // 1200 sec
+
+  int32_t ref = T_REF_INC(pSql->pTscObj);
+  tscDebug("%p add to tscObj:%p, ref:%d", pSql, pSql->pTscObj, ref);
+
+  uint64_t p = (uint64_t) pSql;
+  pSql->self = taosCachePut(tscObjCache, &p, sizeof(uint64_t), &p, sizeof(uint64_t), DEFAULT_LIFE_TIME);
+}
+
 SSqlObj* createSimpleSubObj(SSqlObj* pSql, void (*fp)(), void* param, int32_t cmd) {
   SSqlObj* pNew = (SSqlObj*)calloc(1, sizeof(SSqlObj));
   if (pNew == NULL) {
@@ -1808,8 +1820,7 @@ SSqlObj* createSimpleSubObj(SSqlObj* pSql, void (*fp)(), void* param, int32_t cm
   pNew->sqlstr = strdup(pSql->sqlstr);
   if (pNew->sqlstr == NULL) {
     tscError("%p new subquery failed", pSql);
-
-    free(pNew);
+    tscFreeSqlObj(pNew);
     return NULL;
   }
 
@@ -1820,9 +1831,7 @@ SSqlObj* createSimpleSubObj(SSqlObj* pSql, void (*fp)(), void* param, int32_t cm
 
   tscAddTableMetaInfo(pQueryInfo, pMasterTableMetaInfo->name, NULL, NULL, NULL);
 
-  T_REF_INC(pNew->pTscObj);
-  uint64_t p = (uint64_t) pNew;
-  pNew->self = taosCachePut(tscObjCache, &p, sizeof(uint64_t), &pNew, sizeof(uint64_t), 2 * 600 * 1000);
+  registerSqlObj(pNew);
   return pNew;
 }
 
@@ -2060,10 +2069,7 @@ SSqlObj* createSubqueryObj(SSqlObj* pSql, int16_t tableIndex, void (*fp)(), void
     tscDebug("%p new sub insertion: %p, vnodeIdx:%d", pSql, pNew, pTableMetaInfo->vgroupIndex);
   }
 
-  T_REF_INC(pNew->pTscObj);
-
-  uint64_t p = (uint64_t) pNew;
-  pNew->self = taosCachePut(tscObjCache, &p, sizeof(uint64_t), &pNew, sizeof(uint64_t), 2 * 600 * 10);
+  registerSqlObj(pNew);
   return pNew;
 
 _error:

src/util/inc/tskiplist.h

@@ -136,6 +136,7 @@ typedef struct SSkipListIterator {
   SSkipListNode *cur;
   int32_t        step;          // the number of nodes that have been checked already
   int32_t        order;         // order of the iterator
+  SSkipListNode *next;          // next points to the true qualified node in skip list
 } SSkipListIterator;
 
 /**

src/util/src/tcache.c

@@ -97,7 +97,7 @@ static FORCE_INLINE void taosCacheReleaseNode(SCacheObj *pCacheObj, SCacheDataNo
   int32_t size = (int32_t)taosHashGetSize(pCacheObj->pHashTable);
   assert(size > 0);
 
-  uDebug("cache:%s, key:%p, %p is destroyed from cache, size:%dbytes, num:%d size:%" PRId64 "bytes",
+  uDebug("cache:%s, key:%p, %p is destroyed from cache, size:%dbytes, totalNum:%d size:%" PRId64 "bytes",
          pCacheObj->name, pNode->key, pNode->data, pNode->size, size - 1, pCacheObj->totalSize);
 
   if (pCacheObj->freeFp) {

src/util/src/tskiplist.c

@@ -79,9 +79,12 @@ static SSkipListIterator* doCreateSkipListIterator(SSkipList *pSkipList, int32_t
 
 // when order is TSDB_ORDER_ASC, return the last node with key less than val
 // when order is TSDB_ORDER_DESC, return the first node with key large than val
-static SSkipListNode* getPriorNode(SSkipList* pSkipList, const char* val, int32_t order) {
+static SSkipListNode* getPriorNode(SSkipList* pSkipList, const char* val, int32_t order, SSkipListNode** pCur) {
   __compar_fn_t comparFn = pSkipList->comparFn;
   SSkipListNode *pNode = NULL;
+  if (pCur != NULL) {
+    *pCur = NULL;
+  }
 
   if (order == TSDB_ORDER_ASC) {
     pNode = pSkipList->pHead;
@@ -93,6 +96,9 @@ static SSkipListNode* getPriorNode(SSkipList* pSkipList, const char* val, int32_
           pNode = p;
           p = SL_GET_FORWARD_POINTER(p, i);
         } else {
+          if (pCur != NULL) {
+            *pCur = p;
+          }
           break;
         }
       }
@@ -107,6 +113,9 @@ static SSkipListNode* getPriorNode(SSkipList* pSkipList, const char* val, int32_
           pNode = p;
           p = SL_GET_BACKWARD_POINTER(p, i);
         } else {
+          if (pCur != NULL) {
+            *pCur = p;
+          }
           break;
         }
       }
@@ -295,7 +304,7 @@ SArray* tSkipListGet(SSkipList *pSkipList, SSkipListKey key) {
     pthread_rwlock_wrlock(pSkipList->lock);
   }
 
-  SSkipListNode* pNode = getPriorNode(pSkipList, key, TSDB_ORDER_ASC);
+  SSkipListNode* pNode = getPriorNode(pSkipList, key, TSDB_ORDER_ASC, NULL);
   while (1) {
     SSkipListNode *p = SL_GET_FORWARD_POINTER(pNode, 0);
     if (p == pSkipList->pTail) {
@@ -452,7 +461,7 @@ uint32_t tSkipListRemove(SSkipList *pSkipList, SSkipListKey key) {
     pthread_rwlock_wrlock(pSkipList->lock);
   }
 
-  SSkipListNode* pNode = getPriorNode(pSkipList, key, TSDB_ORDER_ASC);
+  SSkipListNode* pNode = getPriorNode(pSkipList, key, TSDB_ORDER_ASC, NULL);
   while (1) {
     SSkipListNode *p = SL_GET_FORWARD_POINTER(pNode, 0);
     if (p == pSkipList->pTail) {
@@ -545,7 +554,7 @@ SSkipListIterator *tSkipListCreateIterFromVal(SSkipList* pSkipList, const char*
     pthread_rwlock_rdlock(pSkipList->lock);
   }
 
-  iter->cur = getPriorNode(pSkipList, val, order);
+  iter->cur = getPriorNode(pSkipList, val, order, &iter->next);
 
   if (pSkipList->lock) {
     pthread_rwlock_unlock(pSkipList->lock);
@@ -567,8 +576,22 @@ bool tSkipListIterNext(SSkipListIterator *iter) {
   
   if (iter->order == TSDB_ORDER_ASC) {  // ascending order iterate
     iter->cur = SL_GET_FORWARD_POINTER(iter->cur, 0);
+
+    // a new node is inserted into between iter->cur and iter->next, ignore it
+    if (iter->cur != iter->next && (iter->next != NULL)) {
+      iter->cur = iter->next;
+    }
+
+    iter->next = SL_GET_FORWARD_POINTER(iter->cur, 0);
   } else { // descending order iterate
     iter->cur = SL_GET_BACKWARD_POINTER(iter->cur, 0);
+
+    // a new node is inserted into between iter->cur and iter->next, ignore it
+    if (iter->cur != iter->next && (iter->next != NULL)) {
+      iter->cur = iter->next;
+    }
+
+    iter->next = SL_GET_BACKWARD_POINTER(iter->cur, 0);
   }
   
   if (pSkipList->lock) {
@@ -715,8 +738,10 @@ SSkipListIterator* doCreateSkipListIterator(SSkipList *pSkipList, int32_t order)
   iter->order = order;
   if(order == TSDB_ORDER_ASC) {
     iter->cur = pSkipList->pHead;
+    iter->next = SL_GET_FORWARD_POINTER(iter->cur, 0);
   } else {
     iter->cur = pSkipList->pTail;
+    iter->next = SL_GET_BACKWARD_POINTER(iter->cur, 0);
   }
 
   return iter;