Manipulating LOBs Using ADO Recordsets and OLE DB Rowsets

LOB data is never retrieved and stored in the provider cache. When a server cursor is used, OraOLEDB provides the LOB data to the consumer only when it is requested.

Note: Although most LOB columns in an Oracle database support up to 4 GB of data storage, ADO limits the maximum column size to 2 GB.

To incur less round trips to the database, reads and writes should be carried out in large chunks for better performance.

Use Explicit Transactions

When using server cursor in an auto-commit mode, all LOB data modifications are transmitted to the database and committed. This means that even if the recordset is in a deferred update mode, the LOB data modifications and any previous deferred updates, will be permanent. To have flexibility of rolling back LOB data modifications, it is advised that explicit transactions are used when manipulation LOB data.

ADO Recordsets and LOBs

GetChunk()

The GetChunk method of ADO recordset object retrieves LOB data. When subsequent GetChunk() calls are made on the same LOB column, data is retrieved from where it left off. However, if the current row changes or if another LOB column is read from or written to, calling GetChunk() again on the original LOB column will retrieve data from the beginning.

Writing Data to a LOB Column With AppendChunk()

The AppendChunk() method of ADO recordset object writes data to a LOB column. The initial AppendChunk() method will overwrite any existing data. Subsequent AppendChunk() calls will append the data, but the appending will end when the current row changes or when another LOB column data is updated or read from.

OLE DB Rowsets and LOBs

The following OLE DB rowset methods read and write LOB data:

• IRowset::GetData() and ISequentialStream::Read() reads LOB data.

• IRowsetChange::SetData() and ISequentialStream::Write() writes LOB data.