(Someone figured out LIMIT for relational algebra, and ORDER can be coerced to relational if you define a bunch of edge cases. I don't think it's fair to suggest that a RDBMS can't/shouldn't/wouldn't do LIMIT or ORDER).
I agree, relational algebra sucks to code in. That wasn't the point of it, of course. Codd's goal was to prove as long as your language is reducible to relational algebra, you're relational. And with that, you get all the side benefits.
I'm not so sure Date et al are advocating coding purely in RA. At least, I've never heard them say that. Their "Tutorial D", as the name implies, is for educational and specification purposes more than a useable implementation. Again, if you can make your awesome language reducible to "Tutorial D", you get all the nice benefits of the True RDBMS, which is pretty awesome.
I agree, relational algebra sucks to code in. That wasn't the point of it, of course. Codd's goal was to prove as long as your language is reducible to relational algebra, you're relational. And with that, you get all the side benefits.
I'm not so sure Date et al are advocating coding purely in RA. At least, I've never heard them say that. Their "Tutorial D", as the name implies, is for educational and specification purposes more than a useable implementation. Again, if you can make your awesome language reducible to "Tutorial D", you get all the nice benefits of the True RDBMS, which is pretty awesome.