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By changing the COMMIT to ROLLBACK, we can expect a totally different result. The time to roll back is definitely a function of the amount of data modified. I changed the script developed in the previous section to perform a ROLLBACK instead (simply change the COMMIT to ROLLBACK) and the timings are very different. Look at the results now: ops$tkyte%ORA11GR2> declare 2 l_redo number; 3 l_cpu number; 4 l_ela number; 5 begin 6 dbms_output.put_line 7 ( '-' || ' Rows' || ' Redo' || 8 ' CPU' || ' Elapsed' ); 9 for i in 1 .. 6 10 loop 11 l_redo := get_stat_val( 'redo size' ); 12 insert into t select * from big_table where rownum <= power(10,i); 13 l_cpu := dbms_utility.get_cpu_time; 14 l_ela := dbms_utility.get_time; 15 --commit work write wait; 16 rollback; 17 dbms_output.put_line 18 ( '-' || 19 to_char( power( 10, i ), '9,999,999') || 20 to_char( (get_stat_val('redo size')-l_redo), '999,999,999' ) || 21 to_char( (dbms_utility.get_cpu_time-l_cpu), '999,999' ) || 22 to_char( (dbms_utility.get_time-l_ela), '999,999' ) ); 23 end loop; 24 end; 25 / Rows Redo CPU Elapsed 10 3,036 0 0 100 10,632 2 0 1,000 120,532 0 0 10,000 1,212,540 1 1 100,000 12,973,208 9 10 - 1,000,000 130,473,232 124 980 PL/SQL procedure successfully completed. This difference in CPU and Elapsed timings is to be expected, as a ROLLBACK has to physically undo the work we ve done. Similar to a COMMIT, a series of operations must be performed. Before we even get to the ROLLBACK, the database has already done a lot of work. To recap, the following would have happened: Undo segment records have been generated in the SGA. Modified data blocks have been generated in the SGA.

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A buffered redo log for the preceding two items has been generated in the SGA. Depending on the size of the preceding three items and the amount of time spent, some combination of the previous data may be flushed onto disk already. All locks have been acquired.

Summary

When we ROLLBACK, We undo all of the changes made. This is accomplished by reading the data back from the undo segment and, in effect, reversing our operation and then marking the undo entry as applied. If we inserted a row, a ROLLBACK will delete it. If we updated a row, a rollback will reverse the update. If we deleted a row, a rollback will re-insert it again. All locks held by our session are released, and everyone who was enqueued waiting on locks we held will be released.

<Serializable()> _ Public Class Customer Public Function Snapshot() As Byte() Using m As New MemoryStream Dim f As New BinaryFormatter f.Serialize(m, Me) m.Position = 0 return m.ToArray() End Using End Function End Class This converts the object into a byte stream, returning that byte stream as an array of type Byte. That part is easy it s the restoration that s tricky. Suppose that the user now wants to undo the changes, requiring that the byte stream be restored back into the object. The code that deserializes a byte stream looks like this: <Serializable()> _ Public Class Customer Public Function Deserialize(ByVal state As Byte()) As Customer Using m As New MemoryStream(state) Dim f As New BinaryFormatter Return CType(f.Deserialize(m), Customer) End Using End Function End Class Notice that this function returns a new customer object. It doesn t restore the existing object s state; it creates a new object. Somehow, you would have to tell any and all code that has a reference to the existing object to use this new object. In some cases, that might be easy to do, but it isn t always trivial. In complex applications, it s hard to guarantee that other code elsewhere in the application doesn t have a reference to the original object and if you don t somehow get that code to update its reference to this new object, it will continue to use the old one. What s needed is some way to restore the object s state in place, so that all references to the current object remain valid, but the object s state is restored. This is the purpose of the UndoableBase class.

A COMMIT, on the other hand, just flushes any remaining data in the redo log buffers. It does very little work compared to a ROLLBACK. The point here is that you don t want to roll back unless you have to. It is expensive since you spend a lot of time doing the work, and you ll also spend a lot of time undoing the work. Don t do work unless you re sure you are going to want to COMMIT it. This sounds like common sense of course I wouldn t do all of the work unless I wanted to COMMIT it. However, I ve often seen a developer use a real table as a temporary table, fill it up with data, report on it, and then roll back to get rid of the temporary data. Later we ll talk about true temporary tables and how to avoid this issue.

In this chapter, you saw how the functional programming techniques from 3 are often used to implement in-memory queries similar to those used to access databases. You also saw how to use sequence expressions as an alternative notation for these query expressions. We then turned to databases themselves and covered how to use ADO.NET to access relational databases. You also saw how to perform a variety of database-related tasks from Visual Studio. You next saw how to perform simple, typed queries using F# LinqToSql, taking particular advantage of the object/relational data objects generated by the LINQ tool SqlMetal.exe. Finally, you saw how to use XML as a generic data format, partly by using functionality from the LINQ libraries. In the next chapter, we ll cover parsing techniques, including using the lexer and parser generator tools that come with the F# distribution.