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Before you can do any work with a database, you need to establish a connection to it. For instance, you can connect to a locally running instance of SQL Server 2005 Express using the following code: open System.Data open System.Data.SqlClient let connString = @"Server='.\SQLEXPRESS';Integrated Security=SSPI" let conn = new SqlConnection(connString) The value connString is a connection string. Regardless of how you created your connection object, to execute any updates or queries on it you need to open it first:

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You would be right to point out that in this case, if I had not committed on line 10, I would have received the following error: begin * ERROR at line 1: ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDO_SMALL' ORA-06512: at line 6 The major differences between the two errors are as follows: The ORA-01555 example left my update in a totally unknown state Some of the work had been done; some had not There is absolutely nothing I can do to avoid the ORA-01555 error, given that I committed in the cursor FOR loop The ORA-30036 error can be avoided by allocating appropriate resources in the system This error is avoidable by correct sizing; the first error is not.

Further, even if I don t avoid this error, at least the update is rolled back and the database is left in a known, consistent state not halfway through some large update..

> conn.Open();; val it : unit = () If this command fails, then you may need to do one of the following: Install SQL Server 2005 Express or a newer version of the same. Consult the latest SQL Server Express samples for alternative connection strings. Add UserInstance='true' to the connection string. This starts the database engine as a user-level process. Change the connection string if you have a different database engine installed and running (for instance, if you are using SQL Server instead of SQL Server Express). Connections established using the same connection string are pooled and reused depending on your database engine. Connections are often a limited resource and should generally be closed as soon as possible within your application.

The bottom line here is that you can t save on undo space by committing frequently you need that undo. I was in a single-user system when I received the ORA-01555 error. It takes only one session to cause that error, and many times even in real life it is a single session causing its own ORA-01555 errors. Developers and DBAs need to work together to size these segments adequately for the jobs that need to be done. There can be no short-changing here. You must discover, through analysis of your system, what your biggest transactions are and size appropriately for them. The dynamic performance view

V$UNDOSTAT can be very useful to monitor the amount of undo you are generating and the duration of your longest running queries. Many people consider things like temp, undo, and redo as overhead things to allocate as little storage to as possible. This is reminiscent of a problem the computer industry had on January 1, 2000, which was all caused by trying to save 2 bytes in a date field. These components of the database are not overhead, but rather are key components of the system. They must be sized appropriately (not too big and not too small).

Tip The More on Connection Strings sidebar contains more details on creating and managing connection

Note Speaking of UNDO segments being too small, make sure to set your undo tablespace back to your regular one after running these examples, otherwise you ll be hitting ORA-30036 errors for the rest of the book!