> But if it does make sense to share, then that's another reason for not 
> designing something ad-hoc for psql: integrating it later will be more 
> work in total.
Yes, but not much: evaluating "[!] :var" special syntax would be dropped, 
but I do not think that it is the main issue with these features.
I understand that your conclusion is to require psql expressions + 
conditions as one (large) patch.
Given my personnal experience with the patch process, my conclusion is 
that I will very probably not do it. Just adding logical expressions to 
pgbench, a minor feature for a minor client, has already been spread over 
3 CF, and this is a part of what is required for the condition & 
expression in psql.
Hopefully someone else will do it, and I'll do some reviewing then.
-- 
Fabien.