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  1.  Home
  2. RPG400-L
  3. October 2005

Re: Populating fields based on field name in a file

Dynamic SQL requires additional overhead 
> that static SQL does not.  In order to
use SQL with the design model given 
> there is additional I/O to retrieve
all field defaults via RPG(or SQL) 
> to build the SQL statement, then the
execution of the SQL statement 
> that was built.
Understood and agreed. Given all that however, dynamic SQL may prove an adequate performer.
Database (& process) design have 
> more impact on how SQL (or native
> I/O) performs than any other
> thing I can think of.
Absolutely. The thing that many newcomers to SQL (and my remarks are directed in that direction more than anything) tend to do is equate SQL performance to the same metrics they've used for evaluating native I/O. And that just doesn't work very well, because we programmers are very specific about creating a design and a process for specifically telling OS400 _how_ to get the records. In other words, we specifically optimise our code by the way we write it. And that I/O cost scales very linearly until we hit the thrashing point in virtual memory.
SQL is very unlike that. We don't tell SQL _how_ to get the records, we tell it _what_ records we want and then let it go do the work for us. Sometimes it makes decisions that are disastrous in terms of performance, but sometimes it does a great job. In general, newcomers to SQL can't tell how a particular statement will execute on their machine with their dataset. That means that a given SQL statement may scream when there are a hundred records, but drag when there are a thousand. The dataset make-up is crucial to SQL optimisation, and the main influence we have in that regard is the creation of indexes, not by tweaking the SQL code. It's a very different paradigm... 
  --buck

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