Data Access and Transactions

Forward-Only Restrictions on Binary Large Objects

The Binary Large Object (BLOB) is a type of data field that contains large blocks of text, or binary data such as graphics, sound, or compiled code. BLOBs can be fetched with a forward-only or “firehose” cursor, but there are some access restrictions on the resulting Recordset.

• The CacheSize property of the Recordset must be set to 1. If you cache more than a single row, only the BLOB on the last row is available.
• All BLOB columns must be to the right of any scalar (non-BLOB) columns fetched in the query and you must access them in a left-to-right order.

These restrictions have nothing to do with the underlying table layout—only with the Recordset object. For instance, you could have a BLOB column in the middle of your table definition (with scalar columns to the left and right), but when you execute your SELECT statement, you must select the BLOB column after the scalar columns. The storage engine on SQL Server doesn’t impose limitations on where a BLOB column lies, but it is significant to the ODBC driver.

© 1997-1999 Microsoft Corporation. All rights reserved.