Performance of DOMAINs

[David Wheeler <david(at)kineticode(dot)com>]

Howdy,

Didn't see anything in the archives, so I thought I'd ask: has anyone  
done any work to gauge the performance penalty of using DOMAINs? I'm  
thinking of something like Elein's email DOMAIN:

  http://www.varlena.com/GeneralBits/

I figured that most simple domains that have a constraint check are  
no faster or slower than tables with constraints that validate a  
particular column. Is that the case?

But I'm also interested in how Elein made the email domain case- 
insensitive, since I'd like to have/create a truly case-insensitive  
text type (ITEXT anyone?). The functions for the operator class there  
were mainly written in SQL, and if it adds a significant overhead,  
I'm not sure it'd be a good idea to use that approach for a case- 
insensitive text type, since I use it quite a lot in my apps, and  
often do LIKE queries against text data. Thoughts?

Many TIA,

David