從 SQL Server Native Client OLE DB 提供者取得大量數據

適用於： SQL Server Azure SQL 資料庫 Azure SQL 受控執行個體 Azure Synapse Analytics Analytics Platform System (PDW)

一般而言，取用者應該隔離建立 SQL Server Native Client OLE DB 提供者記憶體物件的程式代碼，以及處理未透過 ISequentialStream 介面指標參考之數據的其他程式代碼。

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DBPROP_ACCESSORDER屬性（在數據列集屬性群組中）設定為DBPROPVAL_AO_SEQUENTIAL或DBPROPVAL_AO_SEQUENTIALSTORAGEOBJECTS值之一，取用者應該只會擷取 GetNextRows 方法呼叫中的單一數據列，因為 BLOB 數據不會緩衝處理。 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。

在取用者要求執行此動作之前，SQL Server Native Client OLE DB 提供者不會從 SQL Server 擷取大型數據。 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