The Art of SQL Server FILESTREAM - Red Gate Software

Chapter 2: Getting Started with FILESTREAMA T-SQL batch or stored procedure can access FILESTREAM columns as if they areregular VARBINARY(MAX) columns. However, accessing large BLOB data stored in aFILESTREAM column from T-SQL does not perform well, for the reasons below.• One of the features that FILESTREAM offers is the I/O streaming capability for fasterFILESTREAM operations. When accessing FILESTREAM data through T-SQL, thebenefits of streaming I/O are not available.• When accessing FILESTREAM data using T-SQL, SQL Server memory is usedfor buffering the FILESTREAM data. This will add memory overhead to the SQLServer instance.• To T-SQL, a FILESTREAM column appears as a VARBINARY(MAX) column with a2 GB size limitation. Therefore, if the data stored in the FILESTREAM column islarger than 2 GB, T-SQL can only access the first 2 GB of it.Therefore, for best performance and optimum usage of the SQL Server resources,FILESTREAM data should not generally be accessed using T-SQL. Instead, we shoulduse the streaming APIs exposed by SQL Server, as described in Chapter 3: AccessingFILESTREAM Data from Client Applications.However, as always, there are exceptions to the rule. Most significantly, unless theFILESTREAM column is assigned a default value, then a client application has to executea T-SQL INSERT statement in order to add a new row into a FILESTREAM-enabledtable, rather than use the streaming APIs. The reason for this is that streaming accesscan be made only to rows that contain non-NULL BLOB data in the FILESTREAMcolumn. So, T-SQL INSERT statements on FILESTREAM-enabled tables usually storedummy VARBINARY data in the FILESTREAM column, or a default value can be set onthe FILESTREAM column. For example, we could insert a dummy value, or use a defaultvalue, of 0x. This will create a zero-length file, which can then be replaced by actualFILESTREAM BLOB data using streaming I/O (or T-SQL).71