BTW, (and I appologize if this comes off as condescending... that's not how it's intended) are you familiar with "joins" in relational databases? It's very much at the heart of what makes a relational database "relational". I just ask because a few of the different things you said in this thread make it seem like you're not familiar with using joins. All of the "use a hash table in perl" and "where other_table_id in (select id from other_table where ..." make me wonder.

I ask because, if you're trying to force the issue too much, and avoid using joins where they are appropriate, you could be hamstringing your database a little bit. Most databases have what is known as something along the lines of an "execution path optimizer". That is, you ask the database for information, and the database figures out the best method for retrieving that data. If you descibe the data that you want by using join syntax, the datbase has more freedom to optimize your query. Also, utilizing joins to pull all the info you need in a single query (rather than running multiple queries and joining in perl with hash tables), you can reduce network round-trip time to your database.

Of course, there are always going to be some good excuses to do your joins in perl rather than on the DB... but the default approach should really be with joins.