SQL/RPG

I have a very basic question concerning SQL. Can someone point me to information on how SQL accesses data on the iSeries? What are the underlying mechanics of the process?

Last week I was made aware of a sales related inquiry that was taking many minutes to produce data. The purpose of the inquiry is to display sales summarized by division (12 of them). There are about 1.5 million records in the file, each record is 300 bytes long. There are no logical files built on this physical file.

The program was written using embedded SQL with the fetch option. The program does nothing else but load an array with summary data and then display it using a subfile after all the data is read.

For testing purposes, I also wrote a version using the RPG read option. Both of these options ran about the same time, about 9 minutes. I also tried using the RUNSQLSTM, this ran in 9 seconds.

The time differences are amazing. How does SQL process all this data in less then 9 seconds? I understand there are differences in how SQL and RPG are implemented, but it seems a large difference.

Are there any books, papers, articles, that show how SQL access data on the iSeries? How does it work with DB2, the OS, and the hardware to deliver data?

Thanks,

Dennis