I'm inclined to suggest that we should require all extensions beyond the boolean-literal case to be set up as a keyword followed by appropriate argument(s); that seems like it's enough to prevent syntax conflicts from future additions. So you could imagine
\if defined varname \if sql boolean expression to send to server \if compare value operator value
It would be easy to allow "not" in front of any one of these, but it's less clear how to do AND or OR combinations. You can always fake AND with nested \if's, but OR is a bit more of a problem. Maybe we don't need it.
Other ideas about how to design this?
My 0.02 €:
I have convinced myself that, unlike pgbench, psql does not really need an advanced client-side-implemented language, so the smaller the better. What I mean by this is that from psql point of view it is ok that the actual expression evaluation is performed server-side. From a user experience point of view it would look similar to pgbench, just the evaluator does not need to be client-side.
I am sorry - I disagree - I don't expect hard scripting in psql too. But psql is much more widely used than pgbench - and scripting should be intuitive.
Regards
Pavel
So I would suggest something close but maybe simpler than what you suggest above. If there is just one thing, it is true or false, checked client side, well, this is already implemented:-).
\if something
If there are more than one argument, or maybe if previous true/false evaluation failed, then:
\if sql expression to be evaluated server side
Then the result is checked for true or false client-side. It would be equivalent to:
SELECT sql expression to be evaluted server side AS is_ok \gset \if :is_ok
Finally I would suggest that client to server would only communicate by variable substitution, as the backtick patch with external processes.
For checking variable definition, I would suggest to extend the variable access syntax so that there is no exception to the one thing rule between client side and server side evaluation:
\if :?variable
the :?... is subsituted by true or false depending on whether the variable exists.
\if NOT :?variable
would work by executing "NOT ..." on the server. No need for "defined" which would not look like SQL function calls anyway, no need for any operator client side or clumsy rules.