fix(query): 1. reset the pointer, when initialize the reader failed. 2. add...

fix(query): 1. reset the pointer, when initialize the reader failed. 2. add check for null pointer when extracting cached rows.

source/dnode/vnode/src/tsdb/tsdbCacheRead.c

@@ -41,6 +41,13 @@ static int32_t saveOneRow(SArray* pRow, SSDataBlock* pBlock, SCacheRowsReader* p
         int32_t   slotId = slotIds[i];
         SLastCol* pColVal = (SLastCol*)taosArrayGet(pRow, slotId);
 
+        // add check for null value, caused by the modification of table schema (new column added).
+        if (pColVal == NULL) {
+          p->ts = 0;
+          p->isNull = true;
+          continue;
+        }
+
         p->ts = pColVal->ts;
         p->isNull = !COL_VAL_IS_VALUE(&pColVal->colVal);
         allNullRow = p->isNull & allNullRow;

source/dnode/vnode/src/tsdb/tsdbRead.c

@@ -3942,6 +3942,7 @@ int32_t tsdbReaderOpen(SVnode* pVnode, SQueryTableDataCond* pCond, void* pTableL
 _err:
   tsdbError("failed to create data reader, code:%s %s", tstrerror(code), idstr);
   tsdbReaderClose(pReader);
+  *ppReader = NULL;   // reset the pointer value.
   return code;
 }