Reading BLOB columns from a Database using OLE-DB is pretty simple, there’s an MSDN sample which shows you how to do it.
However, for Oracle BLOB columns, the Oracle OLE-DB provider doesn’t bind the length part of the output column binding in the same way that the SQL-Server OLE-DB provider does.
Oracle sets the length part to 4 bytes, indicating the length of the bind value itself, which is an ISequentialStream* pointer, rather than the size of the BLOB column contents itself.
The SQL-Server OLE-DB provider, instead, sets the length part to the size of the Image column contents. Which is convenient in this case as you can make a single call to the ISequentialStream::Read() method using the length part as the 2nd parameter.
To read the Oracle BLOB column contents, you have to make repeated calls to the ISequentialStream::Read() method using some arbitrary value as the 2nd parameter until the stream is completely read, which invariably involves multiple memory reallocations, as shown below:
ISequentialStream* spStreamToCopyTo = reinterpret_cast<ISequentialStream*>(spSource);
HRESULT hr = S_OK;
ULONG ulBytesRead = 0;
int cbBuf = 0x400000;
BYTE* pBuf = new BYTE[cbBuf];
if(SUCCEEDED(hr = spStreamToCopy->Read(pBuf, cbBuf, &ulBytesRead)))
hr = spStreamToCopyTo->Write(pBuf, ulBytesRead, NULL);
while((cbBuf == ulBytesRead) && SUCCEEDED(hr));