MDAC 2.5 SDK - OLE DB Providers
OLE DB Provider for SQL Server

BLOBs and COM Objects

SQLOLEDB exposes the ISequentialStream interface to support consumer access to Microsoft® SQL Server™ ntext, text, and image data types as binary large objects (BLOBs). The Read method on ISequentialStream allows the consumer to retrieve large amounts of data in manageable chunks.

SQLOLEDB can use a consumer-implemented IStorage interface when the consumer provides the interface pointer in an accessor bound for data modification.

SQLOLEDB Storage Object Limitations

• SQLOLEDB can support only a single open storage object. Attempts to open more than one storage object (attempts to get a reference on more than one ISequentialStream interface pointer) return DBSTATUS_E_CANTCREATE.
  
  
• In SQLOLEDB, the default value of the DBPROP_BLOCKINGSTORAGEOBJECTS read-only property is VARIANT_TRUE. This indicates that if a storage object is active, some methods (other than those on the storage objects) will fail with E_UNEXPECTED.
  
  
• The length of data presented by a consumer-implemented storage object must be made known to SQLOLEDB when the row accessor that references the storage object is created. The consumer must bind a length indicator in the DBBINDING structure used for accessor creation.
  
  
• SQLOLEDB supports the ISequentialStream::Write method for zero-length strings and NULL values only. Attempts to write more than zero bytes through ISequentialStream::Write fail.
  
  
• If a row contains more than a single large data value and if DBPROP_ACCESSORDER is not DBPROPVAL_AO_RANDOM, the consumer must either use a SQLOLEDB cursor-supported rowset to retrieve row data or process all large data values prior to retrieving other row values. If DBPROP_ACCESSORDER is DBPROPVAL_AO_RANDOM, SQLOLEDB caches all the BLOB data so that it can be accessed in any order.