从 SQL Server Native Client OLE DB 访问接口获取大数据

适用于：SQL Server Azure SQL 数据库 Azure SQL 托管实例 Azure Synapse Analytics Analytics Platform System (PDW)

通常，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隔离开来。

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（在行集属性组中）设置为DBPROPVAL_AO_SEQUENTIAL或DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS值之一，则使用者应在调用 GetNextRows 方法时仅提取一行数据，因为 BLOB 数据未缓冲。 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 。

在使用者请求这样做之前，SQL Server Native Client OLE DB 访问接口不会从 SQL Server 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);  
    }  

另请参阅

BLOB 和 OLE 对象
使用大值类型