'Theoretical' SQL question

While reading the thread on SQL and aggregates I had a thought about SQL - well 
really about the underlying optimiser and the SQL engine.

The 'theoretical' SQL question is as follows;

Can one assume 100% that the optimiser/SQL engine will 'break' from processing 
when performing a SELECT * FROM CUSTOMERS WHERE ID = '123456' after hitting the 
record with the matching ID when ID is defined as a unique key field?

I'm in the camp that the software is 'smart enough' (or was written smart 
enough) to realise that once that value is found, and since the key is unique 
there shouldn't/won't be any other records to find that will match the WHERE 
clause, that the processing of the query will terminate right there and then 
and return the found record with no further record searches.

Is this how everyone else would expect the processing to occur?

Also, would it also be expected that the search algorithm (with or without the 
use of indices or access paths) would also be smart enough in this instance 
(selecting on a unique key) that it wouldn't be a traversal search but 
something faster e.g. a binary tree/chop search?

That's my SQL musings for a while :o)

--phil