Fetching BLOB Data Using IRow::GetColumns and ISequentialStream
The following function uses IRow::GetColumns and ISequentialStream to fetch large data:
void InitializeAndExecuteCommand()
{
ULONG iidx;
WCHAR* wCmdString=OLESTR("SELECT * FROM MyTable");
// Do the initialization, create the session, and set command text.
hr = pICommandText->Execute(NULL, IID_IRow, NULL,
&cNumRows,(IUnknown **)&pIRow)))
//Get 1 column at a time.
for(ULONG i=0; i < NoOfColumns; i++)
GetSequentialColumn(pIRow, iidx);
// Do the clean up.
}
HRESULT GetSequentialColumn(IRow* pUnkRow, ULONG iCol)
{
HRESULT hr = NOERROR;
ULONG cbRead = 0;
ULONG cbTotal = 0;
ULONG cColumns = 0;
ULONG cReads = 0;
ISequentialStream* pIStream = NULL;
WCHAR* pBuffer[kMaxBuff]; //50 chars read by ISequentialStream::Read()
DBCOLUMNINFO* prgInfo;
OLECHAR* pColNames;
IColumnsInfo* pIColumnsInfo;
DBID columnid;
DBCOLUMNACCESS column;
hr = pUnkRow->QueryInterface(IID_IColumnsInfo,
(void**) &pIColumnsInfo);
if(FAILED(hr))
goto CLEANUP;
hr = pIColumnsInfo->GetColumnInfo(&cColumns, &prgInfo, &pColNames);
// Get Column ID.
columnid = (prgInfo + (iCol))->columnid;
IUnknown* pUnkStream = NULL;
ZeroMemory(&column, sizeof(column));
column.columnid = prgInfo[iCol].columnid;
// Ask for Iunknown interface pointer.
column.wType = DBTYPE_IUNKNOWN;
column.pData = (LPVOID*) &pUnkStream;
hr = pUnkRow->GetColumns(1, &column);
// Get ISequentialStream from Iunknown pointer retrieved from
// GetColumns().
hr = pUnkStream->QueryInterface(IID_ISequentialStream,
(LPVOID*) &pIStream);
ZeroMemory(pBuffer, kMaxBuff * sizeof(WCHAR));
// Read 50 chars at a time until no more data.
do
{
hr = pIStream->Read(pBuffer, kMaxBuff, &cbRead);
cbTotal = cbTotal + cbRead;
// Process the data.
} while(cbRead > 0);
// Do the cleanup.
return hr;
}