Sparse Columns Support in OLE DB Driver for SQL Server
For more information about sparse column support in OLE DB Driver for SQL Server, Sparse Columns Support (OLE DB).
User Scenarios for Sparse Columns and OLE DB Driver for SQL Server
The following table summarizes the common user scenarios for OLE DB Driver for SQL Server users with sparse columns:
|select * from table or IOpenRowset::OpenRowset.||Returns all columns that are not members of the sparse column_set, plus an XML column that contains the values of all non-null columns that are members of the sparse column_set.|
|Reference a column by name.||The column can be referenced regardless of its sparse column status or column_set membership.|
|Access column_set member columns through a computed XML column.||Columns that are members of the sparse column_set can be accessed by selecting the column_set by name and can have values inserted and updated by updating the XML in the column_set column.
The value must conform to the schema for column_set columns.
|Retrieve metadata for all columns in a table through the DBSCHEMA_COLUMNS schema rowset with no column restriction (OLE DB).||Returns a row for all columns that are not members of a column_set. If the table has a sparse column_set, a row will be returned for it.
Note that this does not return metadata for columns that are members of a column_set.
|Retrieve metadata for all columns, regardless of sparseness or membership in a column_set. This might return a very large number of rows.||Call IDBSchemaRowset::GetRowset for the DBSCHEMA_COLUMNS_EXTENDED schema rowset.|
|Retrieve metadata only for columns that are members of a column_set. This might return a very large number of rows.||Call IDBSchemaRowset::GetRowset for the DBSCHEMA_SPARSE_COLUMN_SET schema rowset.|
|Determine whether a column is sparse.||Consult the SS_IS_SPARSE column of the DBSCHEMA_COLUMNS schema rowset (OLE DB).|
|Determine if a column is a column_set.||Consult the SS_IS_COLUMN_SET column of the DBSCHEMA_COLUMNS schema rowset. Or, consult dwFlags returned by IColumnsinfo::GetColumnInfo or DBCOLUMNFLAGS in the rowset returned by IColumnsRowset::GetColumnsRowset. For column_set columns, DBCOLUMNFLAGS_SS_ISCOLUMNSET will be set.|
|Import and export of sparse columns by BCP for a table with no column_set.||No change in behavior from previous versions of OLE DB Driver for SQL Server.|
|Import and export of sparse columns by BCP for a table with a column_set.||The column_set is imported and exported in the same way as XML; that is, as varbinary(max) if bound as a binary type, or as nvarchar(max) if bound as a char or wchar type.
Columns that are members of the sparse column_set are not exported as distinct columns; they are only exported in the value of the column_set.
|queryout behavior for BCP.||No change in the handling of explicitly named columns from previous versions of OLE DB Driver for SQL Server.
Scenarios involving import and export between tables with different schemas may require special handling.
For more information about BCP, see Bulk Copy (BCP) Support for Sparse Columns, later in this topic.
Down-Level Client Behavior
Down-level clients will return metadata only for columns that are not members of the sparse column_set for SQLColumns and DBSCHMA_COLUMNS.
Down-level clients can access columns that are members of the sparse column_set by name, and the column_set column will be accessible as an XML column to SQL Server 2005 (9.x) clients.
Bulk Copy (BCP) Support for Sparse Columns
There are no changes to the BCP API in OLE DB for the sparse columns or column_set features.
If a table has a column_set, sparse columns are not handled as distinct columns. The values of all sparse columns are included in the value of the column_set, which is exported in the same way as an XML column; that is, as varbinary(max) if bound as a binary type, or as nvarchar(max) if bound as a char or wchar type). On import, the column_set value must conform to the schema of the column_set.
For queryout operations, there is no change to the way explicitly referenced columns are handled. column_set columns have the same behavior as XML columns and sparseness has no effect on the handling of named sparse columns.
However, if queryout is used for export and you reference sparse columns that are members of the sparse column set by name, you cannot perform a direct import into a similarly structured table. This is because BCP uses metadata consistent with a select * operation for the import and is unable to match column_set member columns with this metadata. To import column_set member columns individually, you must define a view on the table that references the desired column_set columns, and you must perform the import operation using the view.