fix(query): set the correct row index during generating join results.

source/libs/executor/src/executorimpl.c

@@ -5500,18 +5500,21 @@ static SSDataBlock* doMergeJoin(struct SOperatorInfo* pOperator) {
 
         int32_t blockId = pExprInfo->base.pParam[0].pCol->dataBlockId;
         int32_t slotId = pExprInfo->base.pParam[0].pCol->slotId;
+        int32_t rowIndex = -1;
 
         SColumnInfoData* pSrc = NULL;
         if (pJoinInfo->pLeft->info.blockId == blockId) {
           pSrc = taosArrayGet(pJoinInfo->pLeft->pDataBlock, slotId);
+          rowIndex = pJoinInfo->leftPos;
         } else {
           pSrc = taosArrayGet(pJoinInfo->pRight->pDataBlock, slotId);
+          rowIndex = pJoinInfo->rightPos;
         }
 
-        if (colDataIsNull_s(pSrc, pJoinInfo->leftPos)) {
+        if (colDataIsNull_s(pSrc, rowIndex)) {
           colDataAppendNULL(pDst, nrows);
         } else {
-          char* p = colDataGetData(pSrc, pJoinInfo->leftPos);
+          char* p = colDataGetData(pSrc, rowIndex);
           colDataAppend(pDst, nrows, p, false);
         }
       }