A more SQL-ish way of doing the same could probably be called COMMAND CONSTRAINTS and look something like this
SELECT ... CHECK (ROWCOUNT BETWEEN 0 AND 1);
It is very near to my proposed ASSERT
Only if the ASSERT syntax would become part of the original statement, it is supposed to check. In Hannu's command constraint example above, the statement that causes the error, and thus will be logged and become identified by the error message, is the actual SELECT (or other DML statement).
this is valid argument.
On second hand, I proposed a ASSERT that was not based on expressions only. There is not a technical issue to write assert with knowledge of related statement.
I think I like the COMMAND CONSTRAINT the best so far.
I not, because when it will not be part of SQL, than parser in plpgsql will be more complex. You have to inject SELECT, UPDATE, INSERT, DELETE