I just discovered, about 4 days ago, that you can format Visio database diagrams in the IDEF1X format. While IDEF1X has many shortcomings, I have to say I find it much easier to understand the relationships and constraints at a quick view in this format rather than the standard format.

Granted I’m still running the 2001 version of Visio for Enterprise Architects, so I’m sure that recent releases have more formats in which to view an object model. However, for the time being, I’m having fun with opening all my old schemas and changing their format.

To get your model in this format, you choose the following menu option:
Database > Options > Document…

Then in the General tab, select IDEF1X as the symbol set.