CommandBehavior.SequentialAccess是否有任何性能提升？

Question

I realized I always read my fields in the order they are returned by index (using constants). So my code is already compatible with CommandBehavior.SequentialAccess as far as i understand.

Would there be any benefits if i turn it on? DataReader is already forward only, read only which is the real performance gain right?

Answer 1

The main usage of this is when you are reading very large CLOB ( nvarchar(max) etc) or BLOB ( varbinary(max) ) fields. In the default usage, it buffers the entire row of data before letting you near it - which could mean it has to allocate a large buffer for any BLOB / CLOB fields. When using sequential mode, it does not buffer the row ; you can use the regular API for small fields (as long as you access them in the correct order), but for the CLOB / BLOB fields you can use the chunk-based APIs ( GetBytes and GetChars ) to access fractions of the data in turn. By doing this you could, for example, process a 40 MB image using only a 1k or 4k buffer.

MSDN says the same

Provides a way for the DataReader to handle rows that contain columns with large binary values. Rather than loading the entire row, SequentialAccess enables the DataReader to load data as a stream. You can then use the GetBytes or GetChars method to specify a byte location to start the read operation, and a limited buffer size for the data being returned.

Answer 2

Yes, there is supposed to be at least some performance gain for using CommandBehavior.SequentialAccess , even when not accessing BLOBs. The Microsoft KB article, An "Invalid attempt to read from column ordinal" error occurs when you use DataReader in Visual C# , states:

Setting the CommandBehavior.SequentialAccess flag causes the DataReader to read both rows and columns sequentially. The rows and columns are not buffered. After you read past a column, it is dropped from memory. Any attempt to re-read the column or read previously read columns results in an exception.

Using the CommandBehavior.SequentialAccess flag provides a performance benefit, especially when using Binary Large Object (BLOB) fields. If you do not use SequentialAccess , all the BLOB data is copied to the client. This can consume a lot of resources.

CommandBehavior.SequentialAccess also improves performance when accessing non-BLOB fields. When CommandBehavior.SequentialAccess is not set, you can access a column out of order; however, you incur the following overhead:

• The column is checked to see if the column is later than a previous accessed column.
• The data for all of the previously accessed columns is retrieved and then cached for potential later retrieval.

Columns must be checked and cached because when you use the DataReader , the underlying stream is forward-only for rows as well as column access.