從 SQL Server Native Client OLE DB 提供者取得大型資料

適用於：SQL ServerAzure SQL DatabaseAzure SQL 受控執行個體Azure Synapse AnalyticsAnalytics Platform System (PDW)

一般而言，取用者應該隔離建立SQL Server Native Client OLE DB 提供者儲存物件的程式碼，與其他處理未透過ISequentialStream介面指標參考之資料的程式碼。

本主題會參考可供下列函數使用的功能：

• IRowset:GetData

• IRow::GetColumns

• ICommand::Execute

如果資料列集屬性群組 (DBPROP_ACCESSORDER屬性) 設定為值DBPROPVAL_AO_SEQUENTIAL或DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS，取用者應該只會擷取 GetNextRows 方法呼叫中的單一資料列，因為 BLOB 資料不會緩衝。 如果 DBPROP_ACCESSORDER 的值設定為 DBPROPVAL_AO_RANDOM，取用者可以在 GetNextRows 中提取資料的多個資料列。

SQL Server Native Client OLE DB 提供者不會從SQL Server擷取大型資料，直到取用者要求這麼做為止。 取用者應該在一個存取子中繫結所有短資料，然後在需要時，使用一或多個暫存的存取子來擷取大型資料值。

範例

此範例會從單一資料行擷取大型資料值：

HRESULT GetUnboundData  
    (  
    IRowset* pIRowset,  
    HROW hRow,  
    ULONG nCol,   
    BYTE* pUnboundData  
    )  
    {  
    UINT                cbRow = sizeof(IUnknown*) + sizeof(ULONG);  
    BYTE*               pRow = new BYTE[cbRow];  
  
    DBOBJECT            dbobject;  
  
    IAccessor*          pIAccessor = NULL;  
    HACCESSOR           haccessor;  
  
    DBBINDING           dbbinding;  
    ULONG               ulbindstatus;  
  
    ULONG               dwStatus;  
    ISequentialStream*  pISequentialStream;  
    ULONG               cbRead;  
  
    HRESULT             hr;  
  
    // Set up the DBOBJECT structure.  
    dbobject.dwFlags = STGM_READ;  
    dbobject.iid = IID_ISequentialStream;  
  
    // Create the DBBINDING, requesting a storage-object pointer from  
    // The SQL Server Native Client OLE DB provider.  
    dbbinding.iOrdinal = nCol;  
    dbbinding.obValue = 0;  
    dbbinding.obStatus = sizeof(IUnknown*);  
    dbbinding.obLength = 0;  
    dbbinding.pTypeInfo = NULL;  
    dbbinding.pObject = &dbobject;  
    dbbinding.pBindExt = NULL;  
    dbbinding.dwPart = DBPART_VALUE | DBPART_STATUS;  
    dbbinding.dwMemOwner = DBMEMOWNER_CLIENTOWNED;  
    dbbinding.eParamIO = DBPARAMIO_NOTPARAM;  
    dbbinding.cbMaxLen = 0;  
    dbbinding.dwFlags = 0;  
    dbbinding.wType = DBTYPE_IUNKNOWN;  
    dbbinding.bPrecision = 0;  
    dbbinding.bScale = 0;  
  
    if (FAILED(hr = pIRowset->  
        QueryInterface(IID_IAccessor, (void**) &pIAccessor)))  
        {  
        // Process QueryInterface failure.  
        return (hr);  
        }  
  
    // Create the accessor.  
    if (FAILED(hr = pIAccessor->CreateAccessor(DBACCESSOR_ROWDATA, 1,  
        &dbbinding, 0, &haccessor, &ulbindstatus)))  
        {  
        // Process error from CreateAccessor.  
        pIAccessor->Release();  
        return (hr);  
        }  
  
    // Read and process BLOCK_SIZE bytes at a time.  
    if (SUCCEEDED(hr = pIRowset->GetData(hRow, haccessor, pRow)))  
        {  
        dwStatus = *((ULONG*) (pRow + dbbinding.obStatus));  
  
        if (dwStatus == DBSTATUS_S_ISNULL)  
            {  
            // Process NULL data  
            }  
        else if (dwStatus == DBSTATUS_S_OK)  
            {  
            pISequentialStream = *((ISequentialStream**)   
                (pRow + dbbinding.obValue));  
  
            do  
                {  
                if (SUCCEEDED(hr =  
                    pISequentialStream->Read(pUnboundData,  
                    BLOCK_SIZE, &cbRead)))  
                    {  
                    pUnboundData += cbRead;  
                    }  
                }  
            while (SUCCEEDED(hr) && cbRead >= BLOCK_SIZE);  
  
            pISequentialStream->Release();  
            }  
        }  
    else  
        {  
        // Process error from GetData.  
        }  
  
    pIAccessor->ReleaseAccessor(haccessor, NULL);  
    pIAccessor->Release();  
    delete [] pRow;  
  
    return (hr);  
    }  

另請參閱

BLOB 與 OLE 物件
使用大型實值型別