After days I am thinking so it can be a good solution
syntax - enhanced current RAISE
RAISE ASSERT WHEN boolean expression
RAISE ASSERT 'some message' WHEN expression
It looks like I'm losing this battle, but this syntax isn't too bad.
and we can have a GUC that controls asserts per database - possibly overwritten by plpgsql option - similar to current plpgsql options
assert_level = [*ignore*, notice, warning, error]
This sounds like a decent enhancement.
Oh, it would be nice to have the option here to say "assertions can't be caught by exception handlers", but I don't know how that mechanism works so I'm not sure it's possible. I'll have to look into that.
Personally, I don't think so it is too important, although it can be nice improvement. I don't see use cases where assert can be handled - and with conditional RAISE we can raise a custom exceptions simply.