fix(query):set null for if the queried table is dropped when retrieving the corresponding tags.

source/libs/executor/src/scanoperator.c

@@ -446,6 +446,16 @@ static STableCachedVal* createTableCacheVal(const SMetaReader* pMetaReader) {
 // const void *key, size_t keyLen, void *value
 static void freeCachedMetaItem(const void* key, size_t keyLen, void* value) { freeTableCachedVal(value); }
 
+
+static void doSetNullValue(SSDataBlock* pBlock, const SExprInfo* pExpr, int32_t numOfExpr) {
+  for (int32_t j = 0; j < numOfExpr; ++j) {
+    int32_t dstSlotId = pExpr[j].base.resSchema.slotId;
+
+    SColumnInfoData* pColInfoData = taosArrayGet(pBlock->pDataBlock, dstSlotId);
+    colDataAppendNNULL(pColInfoData, 0, pBlock->info.rows);
+  }
+}
+
 int32_t addTagPseudoColumnData(SReadHandle* pHandle, const SExprInfo* pExpr, int32_t numOfExpr, SSDataBlock* pBlock,
                                int32_t rows, const char* idStr, STableMetaCacheInfo* pCache) {
   // currently only the tbname pseudo column
@@ -465,14 +475,22 @@ int32_t addTagPseudoColumnData(SReadHandle* pHandle, const SExprInfo* pExpr, int
   SMetaReader mr = {0};
   LRUHandle*  h = NULL;
 
+  // todo refactor: extract method
+  // the handling of the null data should be packed in the extracted method
+
   // 1. check if it is existed in meta cache
   if (pCache == NULL) {
     metaReaderInit(&mr, pHandle->meta, 0);
     code = metaGetTableEntryByUidCache(&mr, pBlock->info.id.uid);
     if (code != TSDB_CODE_SUCCESS) {
+
+      // when encounter the TSDB_CODE_PAR_TABLE_NOT_EXIST error, we proceed.
       if (terrno == TSDB_CODE_PAR_TABLE_NOT_EXIST) {
         qWarn("failed to get table meta, table may have been dropped, uid:0x%" PRIx64 ", code:%s, %s",
               pBlock->info.id.uid, tstrerror(terrno), idStr);
+
+        // append null value before return to caller, since the caller will ignore this error code and proceed
+        doSetNullValue(pBlock, pExpr, numOfExpr);
       } else {
         qError("failed to get table meta, uid:0x%" PRIx64 ", code:%s, %s", pBlock->info.id.uid, tstrerror(terrno),
                idStr);
@@ -498,6 +516,8 @@ int32_t addTagPseudoColumnData(SReadHandle* pHandle, const SExprInfo* pExpr, int
         if (terrno == TSDB_CODE_PAR_TABLE_NOT_EXIST) {
           qWarn("failed to get table meta, table may have been dropped, uid:0x%" PRIx64 ", code:%s, %s",
                 pBlock->info.id.uid, tstrerror(terrno), idStr);
+          // append null value before return to caller, since the caller will ignore this error code and proceed
+          doSetNullValue(pBlock, pExpr, numOfExpr);
         } else {
           qError("failed to get table meta, uid:0x%" PRIx64 ", code:%s, %s", pBlock->info.id.uid, tstrerror(terrno),
                  idStr);