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Posted Oct 4, 2001

Expert One-on-One: Oracle: Pt. 3

By DatabaseJournal.com Staff

By now, you might be able to see where I'm going in this section. I have made references above to other databases and how features are implemented differently in each. With the exception of some read-only applications, it is my contention that building a wholly database-independent application that is highly scalable is extremely hard — and is in fact quite impossible unless you know exactly how each database works in great detail.

For example, let's revisit our initial resource scheduler example (prior to adding the FOR UPDATE clause). Let's say this application had been developed on a database with an entirely different locking/concurrency model from Oracle. What I'll show here is that if you migrate your application from one database to another database you will have to verify that it still works correctly in these different environments.

Let's assume that we had deployed the initial resource scheduler application in a database that employed page-level locking with blocking reads (reads are blocked by writes) and there was an index on the SCHEDULES table:

create index schedules_idx 
      on schedules( resource_name, start_time );

Also consider that the business rule was implemented via a database trigger (after the INSERT had occurred but before the transaction committed we would verify that only our row existed in the table for that time slot). In a page-locking system, due to the update of the index page by RESOURCE_NAME and START_TIME it is very likely that we would have serialized these transactions. The system would have processed these inserts sequentially due to the index page being locked (all of the RESOURCE_NAMEs with START_TIMEs near each other would be on the same page). In that page level locking database our application would be apparently well behaved — our checks on overlapping resource allocations would have happened one after the other, not concurrently.

If we migrated this application to Oracle and simply assumed that it would behave in the same way, we would be in for a shock. On Oracle, which does row level locking and supplies non-blocking reads, it appears to be ill behaved. As we saw previously, we had to use the FOR UPDATE clause to serialize access. Without this clause, two users could schedule the same resource for the same times. This is a direct consequence of not understanding how the database we have works in a multi-user environment.

I have encountered issues such as this many times when an application is being moved from database A to database B. When an application that worked flawlessly in database A does not work, or works in an apparently bizarre fashion, on database B, the first thought is that database B is a 'bad database'. The simple truth is that database B just does it differently — neither database is wrong or 'bad', they are just different. Knowing and understanding how they work will help you immensely in dealing with these issues.

For example, very recently I was helping to convert some Transact SQL (the stored procedure language for SQL Server) into PL/SQL. The developer doing the conversion was complaining that the SQL queries in Oracle returned the 'wrong' answer. The queries looked like this:

  l_some_variable   varchar2(25);
  if ( some_condition )
    l_some_variable := f( ... );
  end if;

  for x in ( select * from T where x = l_some_variable )

The goal here was to find all of the rows in T where X was Null if some condition was not met or where x equaled a specific value if some condition was met.

The complaint was that, in Oracle, this query would return no data when L_SOME_VARIABLE was not set to a specific value (when it was left as Null). In Sybase or SQL Server, this was not the case — the query would find the rows where X was set to a Null value. I see this on almost every conversion from Sybase or SQL Server to Oracle. SQL is supposed to operate under tri-valued logic and Oracle implements Null values the way ANSI SQL requires them to be implemented. Under those rules, comparing X to a Null is neither True or False — it is, in fact, unknown. The following snippet shows what I mean:

ops$tkyte@ORA8I.WORLD> select * from dual;


ops$tkyte@ORA8I.WORLD> select * from dual where null=null;

no rows selected

ops$tkyte@ORA8I.WORLD> select * from dual where null<>null;

no rows selected

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