• 项目

获取大型数据

一般而言，使用者应将创建 SQL Server Native Client OLE DB 访问接口存储对象的代码与处理未通过 ISequentialStream 接口指针引用的数据的其他代码分开。

本主题涉及可用于以下函数的功能：

  • IRowset:GetData

  • IRow::GetColumns

  • ICommand::Execute

如果 DBPROP_ACCESSORDER 属性（位于行集属性组中）设置为值 DBPROPVAL_AO_SEQUENTIAL 或 DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS，由于不缓冲 BLOB 数据，因此使用者在对 GetNextRows 方法的调用中应只提取单行数据。如果 DBPROP_ACCESSORDER 的值设置为 DBPROPVAL_AO_RANDOM，则使用者可以在 GetNextRows 中提取多行数据。

在使用者请求之前，SQL Server Native Client OLE DB 访问接口不会从 SQL Server 中检索大型数据。使用者应在一个取值函数中绑定所有短 (Short) 数据，然后根据需要使用一个或多个临时取值函数检索大型数据值。

示例

本示例从单一列中检索大型数据值：

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);
    }