Could you describe what do you mean? My modules does support work with several databases (different dbh's) at the same time - and should support multiple DB types (MySql, SQLite, Postgree ...). But I have no idea how to possibly join two tables in different DB's (with JOIN or say sub query).

Sometimes one needs to work w/ two different database handles. I can get the data from both, and know that two columns, one from either database, can be used in a join. Only problem is, db1 and db2 know nothing about each other. Hell, they may not even be the same DB. I know the solution to the problem. Get all the data from db1 I need, get all the data from db2 I need. Iterate on one, joining results from the other using this one column.

It doesn't come up often, but it does come up. The module would need to be fed two queries, and two joining column numbers to create a new set of results.

Oh well this isn't perfect world. IMHO that's the job of RDBMS in the first place - they should stick to standards. Only thing I can do is support functions that will be there for as many DB types.

They should but they don't. It doesn't make the problem go away.

Consistency in the low-level operations. I don't trust people's driver level libraries to be remotely consistent. I want to setup, breakdown and have queries adhere to timeouts in a consistent fashion. Setting up evals with alarms is something people should not have to think about.

The only operations I can think of that are common on DBI across multiple DB types is the username and password being setup. That's fine. I have no way of saying, "if a query being executed takes too long, abort and close the statement". DBI handles a lot of this, but not everything. There's an extra few metres left to cover.