[TD-3963]no need to save last row in checkForCachedLastRow

src/tsdb/src/tsdbRead.c

@@ -2860,7 +2860,7 @@ bool tsdbGetExternalRow(TsdbQueryHandleT pHandle) {
 
 /*
  * if lastRow == NULL, return TSDB_CODE_TDB_NO_CACHE_LAST_ROW
- * else set pRes and return TSDB_CODE_SUCCESS
+ * else set pRes and return TSDB_CODE_SUCCESS and save lastKey
  */
 int32_t tsdbGetCachedLastRow(STable* pTable, SDataRow* pRes, TSKEY* lastKey) {
   int32_t code = TSDB_CODE_SUCCESS;
@@ -2872,10 +2872,12 @@ int32_t tsdbGetCachedLastRow(STable* pTable, SDataRow* pRes, TSKEY* lastKey) {
     goto out;
   }
 
+  if (pRes) {
     *pRes = tdDataRowDup(pTable->lastRow);
     if (*pRes == NULL) {
       code = TSDB_CODE_TDB_OUT_OF_MEMORY;
     }
+  }
 
 out:
   TSDB_RUNLOCK_TABLE(pTable);
@@ -2889,7 +2891,6 @@ bool isTsdbCacheLastRow(TsdbQueryHandleT* pQueryHandle) {
 int32_t checkForCachedLastRow(STsdbQueryHandle* pQueryHandle, STableGroupInfo *groupList) {
   assert(pQueryHandle != NULL && groupList != NULL);
 
-  SDataRow pRow = NULL;
   TSKEY    key = TSKEY_INITIAL_VAL;
 
   SArray* group = taosArrayGetP(groupList->pGroupList, 0);
@@ -2900,7 +2901,7 @@ int32_t checkForCachedLastRow(STsdbQueryHandle* pQueryHandle, STableGroupInfo *g
   int32_t code = 0;
   
   if (((STable*)pInfo->pTable)->lastRow) {
-    code = tsdbGetCachedLastRow(pInfo->pTable, &pRow, &key);
+    code = tsdbGetCachedLastRow(pInfo->pTable, NULL, &key);
     if (code != TSDB_CODE_SUCCESS) {
       pQueryHandle->cachelastrow = 0;
     } else {
@@ -2915,7 +2916,6 @@ int32_t checkForCachedLastRow(STsdbQueryHandle* pQueryHandle, STableGroupInfo *g
     pQueryHandle->activeIndex = -1;  // start from -1
   }
 
-  tfree(pRow);
   return code;
 }