Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 8 ■ TRANSACTIONS
This allows you to have robust transactions in the database. These transactions do not have
to be short in duration—they should be exactly as long as they need to be (but no longer).
Transactions are not for the convenience of the computer and its software; they are to protect
your data.
Committing in a Loop
Faced with the task of updating many rows, most programmers will try to figure out some procedural
way to do it in a loop, so that they can commit every so many rows. I’ve heard two
main reasons for doing it this way:
• It is faster and more efficient to frequently commit lots of small transactions than it is to
process and commit one big transaction.
• We don’t have enough undo space.
Both of these conclusions are misguided. Furthermore, committing too frequently leaves
you prone to the danger of leaving your database in an “unknown” state should your update
fail halfway through. It requires complex logic to write a process that is smoothly restartable
in the event of failure. By far the best option is to commit only as frequently as your business
processes dictate and to size your undo segments accordingly.
Let’s take a look at these issues in more detail.
Performance Implications
It is generally not faster to commit frequently—it is almost always faster to do the work in a
single SQL statement. By way of a small example, say we have a table, T, with lots of rows, and
we want to update a column value for every row in that table. We’ll use two tables, T1 and T2,
to demonstrate:
ops$tkyte@ORA10G> create table t1 as select * from all_objects;
Table created.
ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'T1' );
PL/SQL procedure successfully completed.
ops$tkyte@ORA10G> create table t2 as select * from t1;
Table created.
ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'T2' );
PL/SQL procedure successfully completed.
Well, when we go to update, we could simply do it in a single UPDATE statement, like this:
ops$tkyte@ORA10G> set timing on
ops$tkyte@ORA10G> update t1 set object_name = lower(object_name);
48306 rows updated.
Elapsed: 00:00:00.31