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CHAPTER 1 DEVELOPING SUCCESSFUL ORACLE APPLICATIONS
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Note I will use autonomous transactions throughout this book to demonstrate locking, blocking, and
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concurrency issues. It is my firm belief that autonomous transactions are a feature that Oracle should not have exposed to developers for the simple reason that most developers do not know when and how to use them properly. The improper use of an autonomous transaction can and will lead to logical data-integrity corruption issues. Beyond using them as a demonstration tool, autonomous transactions have exactly one other use as an error-logging mechanism. If you wish to log an error in an exception block, you need to log that error into a table and commit it without committing anything else. That would be a valid use of an autonomous transaction. If you find yourself using an autonomous transaction outside the scope of logging an error or demonstrating a concept, you are almost surely doing something very wrong.
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Here, I will use an autonomous transaction in the database to have two concurrent transactions in a single session. An autonomous transaction starts a subtransaction separate and distinct from any already established transaction in the session. The autonomous transaction behaves as if it were in an entirely different session for all intents and purposes, the parent transaction is suspended. The autonomous transaction can be blocked by the parent transaction (as we ll see) and, further, the autonomous transaction can t see uncommitted modifications made by the parent transaction. For example: ops$tkyte%ORA11GR2>>> create table t 2 ( processed_flag varchar2(1) 3 ); Table created. ops$tkyte%ORA11GR2> create bitmap index 2 t_idx on t(processed_flag); Index created. ops$tkyte%ORA11GR2> insert into t values ( 'N' ); 1 row created. ops$tkyte%ORA11GR2> declare 2 pragma autonomous_transaction; 3 begin 4 insert into t values ( 'N' ); 5 commit; 6 end; 7 / declare * ERROR at line 1: ORA-00060: deadlock detected while waiting for resource ORA-06512: at line 4
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CHAPTER 1 DEVELOPING SUCCESSFUL ORACLE APPLICATIONS
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Since I used an autonomous transaction and created a subtransaction, I received a deadlock meaning my second insert was blocked by my first insert. Had I used two separate sessions, no deadlock would have occurred. Instead, the second insert would have just blocked and waited for the first transaction to commit or roll back. This symptom is exactly what the project in question was facing the blocking, serialization issue. So we had an issue whereby not understanding the database feature (bitmap indexes) and how it worked doomed the database to poor scalability from the start. To further compound the problem, there was no reason for the queuing code to ever have been written. The database has built-in queuing capabilities and has had them since version 8.0 of Oracle which was released in 1997. This built-in queuing feature gives you the ability to have many producers (the sessions that insert the N, the unprocessed records) concurrently put messages into an inbound queue and have many consumers (the sessions that look for N records to process) concurrently receive these messages. That is, no special code should have been written in order to implement a queue in the database. The developers should have used the built-in feature. And they might have, except they were completely unaware of it. Fortunately, once this issue was discovered, correcting the problem was easy. We did need an index on the processed-flag column, just not a bitmap index. We needed a conventional B*Tree index. It took a bit of convincing to get one created. No one wanted to believe that conventionally indexing a column with two distinct values was a good idea. But after setting up a simulation (I am very much into simulations, testing, and experimenting), we were able to prove it was not only the correct approach but also that it would work very nicely. When we created the index, we had to choose between the following approaches: Just create an index on the processed-flag column. Create an index only on the processed-flag column when the processed flag is N, that is, only index the values of interest. We typically don t want to use an index when the processed flag is Y since the vast majority of the records in the table have the value Y. Notice that I did not say We never want to use . You might want to very frequently count the number of processed records for some reason, and then an index on the processed records might well come in very handy.
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In the chapter on indexing, we ll go into more detail on both types. In the end, we created a very small index on just the records where the processed flag was N. Access to those records was extremely fast and the vast majority of Y records did not contribute to this index at all. We used a function-based index on a function decode( processed_flag, 'N', 'N' ) to return either N or NULL since an entirely NULL key is not placed into a conventional B*Tree index, we ended up only indexing the N records. Was that the end of the story No, not at all. My client still had a less than optimal solution on its hands. They still had to serialize on the dequeue of an unprocessed record. We could easily find the first unprocessed record quickly using select * from queue_table where decode( processed_flag, 'N', 'N') = 'N' FOR UPDATE, but only one session at a time could perform that operation. The project was using Oracle 10g and therefore could not yet make use of the relatively new SKIP LOCKED feature added in Oracle 11g Release 1. SKIP LOCKED would permit many sessions to concurrently find the first unlocked, unprocessed record, lock that record, and process it. Instead, we had to implement code to find the first unlocked record and lock it manually. Such code would in general look like the following in Oracle 10g and before. We begin by creating a table with the requisite index described above and populate it with some data: ops$tkyte%ORA11GR2> create table t 2 ( id number primary key, 3 processed_flag varchar2(1), 4 payload varchar2(20) 5 );
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