BLOBs and OLE Objects
The SQL Native Client OLE DB provider exposes the ISequentialStream interface to support consumer access to SQL Server ntext, text, image, varchar(max), nvarchar(max), varbinary(max), and xml data types as binary large objects (BLOBs). The Read method on ISequentialStream lets the consumer retrieve much data in manageable chunks.
The SQL Native Client OLE DB provider can use a consumer-implemented IStorage interface when the consumer provides the interface pointer in an accessor bound for data modification.
For large value data types, the SQL Native Client OLE DB provider checks for type size assumptions in IRowset and DDL interfaces. Columns with varchar, nvarchar, and varbinary data types with max size set to unlimited will be represented as ISLONG through the schema rowsets and interfaces returning column data types.
The SQL Native Client OLE DB provider exposes the varchar(max), varbinary(max) and nvarchar(max) types as DBTYPE_STR, DBTYPE_BYTES and DBTYPE_WSTR respectively.
To work with these types an application has the following options:
- Bind as the type (DBTYPE_STR, DBTYPE_BYTES, DBTYPE_WSTR). If the buffer is not big enough truncation will occur, exactly as for these types in previous releases (although larger values are now available).
- Bind as the type and also specify DBTYPE_BYREF.
- Bind as DBTYPE_IUNKNOWN and use streaming.
If bound to DBTYPE_IUNKNOWN, ISequentialStream stream functionality is used. The SQL Native Client OLE DB provider supports binding output parameters as DBTYPE_IUNKNOWN for large value data types to facilitate scenarios where a stored procedure returns these data types as return values which will be exposed as DBTYPE_IUNKNOWN to the client.
For additional information, see,
Storage Object Limitations
- The SQL Native Client OLE DB provider can support only a single open storage object. Attempts to open more than one storage object (to get a reference on more than one ISequentialStream interface pointer) return DBSTATUS_E_CANTCREATE.
- In the SQL Native Client OLE DB provider, 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 the SQL Native Client OLE DB provider 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.
- If a row contains more than a single large data value and DBPROP_ACCESSORDER is not DBPROPVAL_AO_RANDOM, the consumer must either use a SQL Native Client OLE DB provider cursor-supported rowset to retrieve row data or process all large data values before retrieving other row values. If DBPROP_ACCESSORDER is DBPROPVAL_AO_RANDOM, the SQL Native Client OLE DB provider caches all the xml data types as binary large objects (BLOBs) so that it can be accessed in any order.
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概念
SQL Native Client (OLE DB)
Using Large Value Types