PHP/SQL/DB2 on IBM i – Scalability?

I’m working on an enterprise software application for a DB2 database on an AS400 system.

I’ve written it in PHP mostly because of the advantages it gives for outputting data to HTML (the application exists to put the data into a company-specific format). It’s effective, but I’m worried about scalability. I devoted a large portion of the development cycle to ensuring everything was properly secured, and I want to make sure that I don’t overlook the performance side of things.

I know that the system will grow slower and slower as the database gets larger. Each time a document is accessed, the program sends a new SQL query to fetch the data – right now, with several hundred documents (each with data across several tables), the data is fetched rapidly (in milliseconds). Soon enough, the system will have thousands of documents, and I want to make sure everything continues to execute quickly (especially as things get into the millions and billions a few years down the line).

I’m reading through IBM’s own performance advice for the system we’re using, but I’d be interested to hear advice from anyone who’s worked with a similar environment before. My guess is that all I really need to do is to make sure the queries (and particularly the WHERE clauses and any JOINs) are well optimized (I’ll start collecting performance data soon).

I’m probably worrying a bit too much, and I know writing code for a problem I don’t fully understand yet is a recipe for disaster. Want to know if anyone has any tips, advice, or recommended reading.

Thanks!